tests/run-make/crate-loading: Rename source files for clarity
For rust-lang/rust#146874 I originally tried to extend the existing test **tests/run-make/crate-loading**. That didn't work out since adding a re-export of the entire crate significantly changes the emitted error messsage.
I did put some effort into making that test easier to understand however, by renaming its files. (Since I was confused myself at first.) Let's save some time for future devs by doing just the renames.
Further cleanups are possible, but that will change the blessed output, so let's not do that right now.
r? ```@jieyouxu``` since you have the context of rust-lang/rust#146874
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