Use correct plugin_kind in user file sources

[davelopez]

Extracted from #19619 to simplify the review

Forcing every user file source to rfs was ok until #19619
Using the correct plugin_kind enables other kinds of file sources like rdm to work as they are supposed and not limited to regular file source logic.

How to test the changes?

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[jmchilton]

Makes good sense to me - thanks for this!