packages: allow idempotent exports_files visibility

[abishekgiri]

Summary

This change makes repeated exports_files() declarations with the same logical visibility idempotent, even when the visibility is represented by distinct RuleVisibility instances.

What changed

• compare exported-file visibility with logical equality instead of object identity
• add regression coverage for calling exports_files() twice with the same custom package visibility

Root cause

exports_files() used != to compare visibilities. That works for singleton public and private visibilities, but it rejects repeated custom visibilities because each parse can allocate a fresh RuleVisibility object even when the declared labels are identical.

Impact

Packages can safely repeat the same custom visibility in exports_files() without tripping a false "declared twice" error, while genuinely conflicting visibilities still fail.

Testing

Tried locally:

• bazel test //src/test/java/com/google/devtools/build/lib/packages:PackagesTests --test_filter='.testExportTwice(CustomVisibilityOK|PublicOK|Fail).' --test_output=errors

Current local blocker:

• analysis completed, but execution on this machine is blocked by an unaccepted Xcode license (xcrun failed with code 69 from apple_support)

[abishekgiri]

Reopened this change from the neutral branch name exports-files-visibility-fix. The targeted test command is in the PR body; local execution on this machine is still blocked by the unaccepted Xcode license.