Backport of #25287.

Closes gh-25286. We don't have tests for .pyf.src files, which can be added in a follow up. It would also be nice for f2py to provide the from_template functionality instead of forcing generate_f2pymod.py in scipy to vendor it from distutils.

@tylerjereddy if this works for you as well I'd suggest it might be worthwhile to merge this now.

@charris this will need to be backported.

A lot of these are very annoyingly tiny bugs, but each one slowly inches f2py towards a better testsuite (among other things).

This PR:

• Don't lower .pyf files
• Stop making useless untitled modules
• Fix some of the #defines

Also:

• Vendors the .src file handling
• Re-establishes old distutils behavior w.r.t. compiled extension modules
  • Basically overwrites them

Tests:

• Ran scipy builds locally
• Added tests for both kinds of character handling (pre-ENH: Support character and character string arrays #19388 and post)