RuleContext.checkSrcsSamePackage has no callers (dead code)

## Summary

`RuleContext.checkSrcsSamePackage(boolean onlyWarn)` is a public method that validates cross-package `srcs` references and emits either an `attributeWarning` or `attributeError`. As of the current tree, **nothing calls it**.

## Location

`src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java` (around line 1300):

```java
/**
* Check that all targets that were specified as sources are from the same package as this rule.
* Output a warning or an error for every target that is imported from a different package.
*/
public void checkSrcsSamePackage(boolean onlyWarn) {
...
if (onlyWarn) {
attributeWarning("srcs", message);
} else {
attributeError("srcs", message);
}
}
```

A repo-wide search finds only the method definition — no call sites in production code, tests, or Starlark rules.

## History

The method previously had callers, but they were removed over time:

1. **2019** — Removed from `CcLibrary` and `CcBinary` intentionally (closes #10121, RELNOTES: "Remove 'please do not import' warning for cc rules"). Commit `5d4e855fd0`.
2. **Later** — Removed along with native rule implementations when rules migrated out of Bazel core:
- `JavaBinary` (commit `f551289281`)
- native `java_library` (commit `1192addd99`)
- Android rules (`android_local_test` base class, etc.)

After those removals, the helper was left behind with no remaining callers.

## Impact

- The cross-package `srcs` diagnostic path is unreachable.
- `attributeWarning("srcs", ...)` from this method can never fire.
- Maintainers may assume this check is active when it is not.

## Suggested next steps

Please pick one (or clarify intent):

1. **Remove the dead code** if cross-package `srcs` warnings/errors are no longer desired.
2. **Re-wire the check** centrally (e.g. in prerequisite validation) if the policy is still useful — possibly as a warning, matching the old `onlyWarn=true` behavior.
3. **Document explicitly** if the method is kept intentionally for external/custom rule authors (though it is currently unused even in-tree).

Happy to send a PR once direction is confirmed.

Start with checkSrcsSamePackage in src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java and verify the reported repo-wide search for callers. Review the linked history and wait for a decision on removal, rewiring, or documentation before changing anything. Done means implementing the chosen direction and updating relevant tests or documentation.

Written by the indexing model from the issue text.