WIP: Match `alias` targets with `register_toolchain` `:all` patterns

[fmeum]

register_toolchains("//pkg:all") now follows alias targets defined in pkg and register any toolchains they point to.

Also moves the toolchain targets defined in @bazel_tools//tools/jdk to a separate package to realize the recommendation given in the release notes below and thus prevent eager fetches of JDKs for unused toolchains.

RELNOTES: :all target patterns provided in register_toolchains calls now also match alias in addition to toolchain targets. To avoid unnecessary fetches, toolchain targets should not be declared in packages with alias targets referencing repositories that aren't meant to be fetched unconditionally.

Fixes #16298