SOLR-15951: Local packages

[chatman]

https://issues.apache.org/jira/browse/SOLR-15951

Ability to load packages from the filesystem, useful for first party packages.

TODO: Generate the local_packages.json on the fly (based on the Solr's version that is being built).

[chatman]

I had some issues rebasing to latest main around the gradle part (due to the "contrib" vs "modules" switch). I'll give it a try again shortly. FYI @noblepaul.

[chatman]

@HoustonPutman Gradle changes look and work great :-)

[chatman]

@HoustonPutman I tried to merge main into this branch, rebase on main, as well as apply these changes on a fresh branch forked from main.

On every occasion, the doLast task in solr/modules/build.gradle is not executing at all, and hence the local_packages.json isn't getting generated. Here's a branch based on fresh main. https://github.com/apache/solr/tree/jira/solr-15951. Any idea, please?

[HoustonPutman]

Ok I merged main in and got stuff working. Haven't really tested everything, but I did test that the packaging works, and the the local_packages.json and the manifest.json files get created.

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