Promote symfony/finder from dev-dependency to full dependency

Problem/Motivation

symfony/finder is currently a dev-dependency of Drupal core, which allows its use during the development of core, but not during actual site development. The component is not included in either the tarball or composer versions of Drupal

This issue aims to promote the dependency out of dev status, making those components available to parts of core Drupal developers may use in their projects.

Issues/Projects Impacted

#3364885: Allow themes to use a starterkit.yml file so it is easier to use the theme generator aims to use symfony/finder to locate files using the starterkit theme's machine name or label in their filename or contents, rather than creating our own solution to recursively scan theme directories

The automatic updates initiative recently removed finder as a dependency in #3321933: Remove dependency on symfony/finder, but @phenaproxima said they would surely use it if were a core dependency (slack link).

Background

In slack, @catch says:

The main thing with both is that they've got overlap, but not interchangeable, with subsystems we've already got in core, so ideally we'd identify where that overlap is and isn't, and what should and shouldn't be converted etc. - it doesn't need to be fully done but it'd be good to have a start considering the last time this was looked at was ten years ago for finder.

Also that's probably the only thing, the normal dependency evaluation stuff is covered by 'it's another Symfony component' so it's just making sure we know where the lines are between it and existing code. We have the same overlap issues with the translation component (which we've been careful to avoid dependencies on) and other code.

#1451320: Evaluate Symfony2's Finder Component to simplify file handling is 13 years old but would be somewhere to start.

If the answer is 'no we shouldn't use any of it', that's fine too but it should go in a change record or something.

Next Steps

1. Need someone with PHP expertise evaluate symfony/finder for additional potential usage outside of its use for starterkit
2. If there are no additional potential usages, we need to make sure that's reflected in the change record