Allow globbing for C files

Right now C files need to be selected one-by-one. This can be inconvenient, especially if such files are automatically generated (as we do in [inline-c](https://github.com/fpco/inline-c)). It would be convenient to be able to glob them, or at least specify a directory instead of each file.

Start by tracing how Cabal currently selects C files and where individual file paths are parsed. Support glob patterns or directory specifications, then verify that generated C files can be selected without listing each file individually; the issue does not name a specific source file or test.

Written by the indexing model from the issue text.