[9.2.0] Gate external-repo source dir package boundary checks behind a flag

[bazel-io]

Forked from #30019

[bazel-io]

Cherry-pick was attempted but there were merge conflicts in the following file(s). Please resolve manually.

src/main/java/com/google/devtools/build/lib/packages/semantics/BuildLanguageOptions.java

cc: @bazelbuild/triage

[Wyverald]

#30040