phpstan fails for a single file ending with 'api.php'

Problem/Motivation

This was discovered in #3393955-7: Correct $suggestions documentation in hook_theme_suggestions_alter() → when using the commit-code-check script.

I then mucked around, and reported this in committer slack.

Looks like a phpstan problem when a single file is to be scanned. Using phpstan inside ddev it fails with any one *.api.php file but if another file is on the command line it works. It does successfully find a single file when it is theme.ap.php or theme.a.php.

And adding api.php to the fileExtensions had no effect.

Steps to reproduce

TBD

Remaining tasks

TBD

User interface changes

API changes

Data model changes

Release notes snippet