🏗 Lint OWNERS files with gulp lint

[rsimha]

This PR makes it so that gulp lint, gulp lint --local_changes, and gulp lint --files=<glob> are all capable of linting OWNERS files.

Example usage:

$ gulp lint --files=build-system/**/OWNERS
[16:47:00] Using gulpfile ~/src/amphtml/gulpfile.js
[16:47:00] Starting 'lint'...
[16:47:00] All packages in node_modules are up to date.
[16:47:00] INFO: Running lint on the following files:
[16:47:00] build-system/global-configs/OWNERS
[16:47:00] build-system/OWNERS
[16:47:01] SUCCESS: No linter warnings or errors.
[16:47:01] Finished 'lint' after 816 ms

$ gulp lint --local_changes
[16:47:45] Using gulpfile ~/src/amphtml/gulpfile.js
[16:47:45] Starting 'lint'...
[16:47:45] All packages in node_modules are up to date.
[16:47:45] INFO: Running lint on the following files:
[16:47:45] build-system/OWNERS
[16:47:45] build-system/tasks/lint.js
[16:47:46] SUCCESS: No linter warnings or errors.
[16:47:46] Finished 'lint' after 1 s

Follow up to #24938

[estherkim]

Dang 2 min too late.

[rsimha]

Dang 2 min too late.

Good catch, but this was an intentional change improved by your feedback :)