Add a git merge driver for `MODULE.bazel.lock`

[fmeum]

Adds a jq script to scripts/ that merges any number of MODULE.bazel.lock files without using Bazel or reading the corresponding MODULE.bazel files.

The lockfile docs now have a section explaining the steps needed to set up this script as a custom merger driver for Git, which means that merge conflicts in MODULE.bazel.lock files will always be resolved automatically. Note that resolution may emit lockfiles with redundant information that will be dropped by subsequent Bazel invocations.

When Bazel encounters an error during lockfile parsing that could be caused by a merge conflict, it emits a different error message with a link to the docs. This required fixing the following kind of server crash when a conflict marker occurs inside a recordedFileInputs object:

FATAL: bazel crashed due to an internal error. Printing stack trace:
java.lang.RuntimeException: Unrecoverable error while evaluating node 'com.google.devtools.build.lib.bazel.bzlmod.BazelLockFileValue$$Lambda/0x000000f8011da998@314cd9ee' (requested by nodes 'RegistryKey{url=https://bcr.bazel.build/}')
	at com.google.devtools.build.skyframe.AbstractParallelEvaluator$Evaluate.run(AbstractParallelEvaluator.java:557)
	at com.google.devtools.build.lib.concurrent.AbstractQueueVisitor$WrappedRunnable.run(AbstractQueueVisitor.java:426)
	at java.base/java.util.concurrent.ForkJoinTask$AdaptedRunnableAction.exec(ForkJoinTask.java:1403)
	at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:387)
	at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1312)
	at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1843)
	at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1808)
	at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:188)
Caused by: java.lang.IllegalArgumentException: the provided path should be absolute in the filesystem
	at com.google.common.base.Preconditions.checkArgument(Preconditions.java:143)
	at com.google.devtools.build.lib.rules.repository.RepoRecordedInput$RepoCacheFriendlyPath.createOutsideWorkspace(RepoRecordedInput.java:202)
	at com.google.devtools.build.lib.rules.repository.RepoRecordedInput$RepoCacheFriendlyPath.parse(RepoRecordedInput.java:222)
	at com.google.devtools.build.lib.rules.repository.RepoRecordedInput$File$1.parse(RepoRecordedInput.java:265)
	at com.google.devtools.build.lib.bazel.bzlmod.GsonTypeAdapterUtil$11.read(GsonTypeAdapterUtil.java:376)
	at com.google.devtools.build.lib.bazel.bzlmod.GsonTypeAdapterUtil$11.read(GsonTypeAdapterUtil.java:367)
	at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.read(TypeAdapterRuntimeTypeWrapper.java:41)
	at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.read(MapTypeAdapterFactory.java:186)
	at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.read(MapTypeAdapterFactory.java:145)
	at com.google.devtools.build.lib.bazel.bzlmod.DelegateTypeAdapterFactory$1.read(DelegateTypeAdapterFactory.java:133)
	at com.google.devtools.build.lib.bazel.bzlmod.LockFileModuleExtension_GsonTypeAdapter.read(LockFileModuleExtension_GsonTypeAdapter.java:171)
	at com.google.devtools.build.lib.bazel.bzlmod.LockFileModuleExtension_GsonTypeAdapter.read(LockFileModuleExtension_GsonTypeAdapter.java:17)
	at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.read(TypeAdapterRuntimeTypeWrapper.java:41)
	at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.read(MapTypeAdapterFactory.java:187)
	at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.read(MapTypeAdapterFactory.java:145)
	at com.google.devtools.build.lib.bazel.bzlmod.DelegateTypeAdapterFactory$1.read(DelegateTypeAdapterFactory.java:133)
	at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.read(TypeAdapterRuntimeTypeWrapper.java:41)
	at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.read(MapTypeAdapterFactory.java:187)
	at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.read(MapTypeAdapterFactory.java:145)
	at com.google.devtools.build.lib.bazel.bzlmod.DelegateTypeAdapterFactory$1.read(DelegateTypeAdapterFactory.java:133)
	at com.google.devtools.build.lib.bazel.bzlmod.BazelLockFileValue_GsonTypeAdapter.read(BazelLockFileValue_GsonTypeAdapter.java:129)
	at com.google.devtools.build.lib.bazel.bzlmod.BazelLockFileValue_GsonTypeAdapter.read(BazelLockFileValue_GsonTypeAdapter.java:15)
	at com.google.gson.Gson.fromJson(Gson.java:991)
	at com.google.gson.Gson.fromJson(Gson.java:956)
	at com.google.gson.Gson.fromJson(Gson.java:905)
	at com.google.gson.Gson.fromJson(Gson.java:876)
	at com.google.devtools.build.lib.bazel.bzlmod.BazelLockFileFunction.getLockfileValue(BazelLockFileFunction.java:93)
	at com.google.devtools.build.lib.bazel.bzlmod.BazelLockFileFunction.compute(BazelLockFileFunction.java:73)
	at com.google.devtools.build.skyframe.AbstractParallelEvaluator$Evaluate.run(AbstractParallelEvaluator.java:468)
	... 7 more

Alternatives considered:

• Letting Bazel resolve the conflict would require building knowledge about particular version control systems and their conflict style into Bazel. It would also either require the user to resolve conflicts in MODULE.bazel first or deviate from the current behavior that the lockfile is not updated when any Bzlmod error is encountered. The jq script can be used as is by every VCS with merge driver support and resolves the conflict in MODULE.bazel.lock independently of MODULE.bazel.
• Implementing the git merge driver as a bazel mod subcommand. This could be the source of intransparent slowdowns during regular git operations, which may even be triggered by other tools such as IDEs. The jq script is very fast.
• Implementing the merger as a Go binary in buildtools would replace the ubiquitous jq tool with a special purpose binary while also not solving the problem that per-user action is required once to register a custom merge driver.

Implements https://docs.google.com/document/d/1TjA7-M5njkI1F38IC0pm305S9EOmxcUwaCIvaSmansg/edit#heading=h.5mcn15i0e1ch

RELNOTES: Git merge conflicts in MODULE.bazel.lock files can be resolved automatically. See https://bazel.build/external/lockfile#automatic-resolution for the required setup.