--incompatible_check_external_repo_source_dir_package_boundary

### Motivation

The check that prevents source directories from being allowed to traverse into another package was affected by a bug that made it a no-op in almost all cases in Bazel 9, except for some false positives that were fixed in 9.2.0 (https://github.com/bazelbuild/bazel/issues/29688). In order to keep this fix backwards compatible, the check was fully disabled for external repos in https://github.com/bazelbuild/bazel/pull/30019. As crossing packages within a source directory can violate visibility constraints and create otherwise impossible combinations of inputs, it is now being enabled as an incompatible change.

### Description

When enabled, a source directory in an external repository is no longer allowed to traverse a package boundary, i.e., it must not contain files that lie in a subpackage (a subdirectory with a BUILD file).

### Incompatible Flag

--incompatible_check_external_repo_source_dir_package_boundary

### Migration Guide

Restructure packages so that source directories do not traverse into subpackages.

### In which Bazel LTS version will this incompatible change be enabled?

Bazel 10

### Additional Context

_No response_

### TODO List

_No response_