Auto merge of #4496 - rwakulszowa:infer_from_subdirectories, r=matklad

Infer targets from subdirectories

Fixes #4086

I still have a few questions:
- should I add some tests for the old behaviour? It isn't really tested at the moment (no tests failed when I broke the implementation); I could refactor the tests to check for both single file and subdirectory inference
- I moved things around, mostly reusing the code from `inferred_bins` - hopefully I didn't break anything, but it won't hurt to double check :)
- Do we have something like servo's `tidy` check for coding style? I'm open for suggestions if something isn't formatted correctly
- Just a general one - should I rebase + squash commits every time I make subsequent changes to cargo?