Try to find modules in PACKAGE.jl as well

This change adds a case to `find_in_path` that attempts to find
directories appended with ".jl" as well.

The trend seems to be naming package repositories as PACKAGE.jl, but you
then had to clone packages to a local directory stripped of the ".jl" so
that files could be found.

No more!
Clean up `find_in_path`

Use `joinpath` instead of `string` for building paths and string slicing
instead of regex matching to remove ".jl" from the name.
Merge pull request #2880 from zachallaun/require-jl

Try to find modules in PACKAGE-NAME.jl as well