Fix mindev ruletype lint.

[blkt]

Summary

We recently changed the behaviour of minder ruletype create and apply to ignore files that do not define rule types. This allowed us to add test files to mindersec/minder-rules-and-profiles, but it in turn broke mindev.

This change makes mindev behave like minder by splitting folder walk from ruletype validation.

Fixes #4819

Change Type

Mark the type of change your PR introduces:

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

Manually ran the following commands pre- and post-fix.

mindev ruletype lint -r ~/stacklok/minder-rules-and-profiles/rule-types/ --skip-rego
mindev ruletype lint -r ~/stacklok/minder-rules-and-profiles/rule-types/common/license.yaml --skip-rego
minder ruletype create -f ~/stacklok/minder-rules-and-profiles/rule-types/

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.