Issue on non-case sensitive filesystems: `sources` and `SOURCES`

`./sources` and `./SOURCES` do not play well on non-case sensitive filesystems (such as macOS's out-of-the-box configuration).

This also means that on macOS, a checkout cannot be "not dirty".

Would it be possible to consider using filenames which don't clash on non-case sensitive filesystems?

Thank you!

Start by reproducing the checkout and dirty-state problem with ./sources and ./SOURCES on a non-case-sensitive filesystem such as the stated macOS configuration. Trace where these paths are created or referenced, then verify that the chosen non-clashing filenames allow a clean checkout on that filesystem.

Written by the indexing model from the issue text.