Make visible and hidden libloc files separate

[Forestryks]

Change libloc arguments from -libloc <dir>:<libs>:<hidden_libs> to -I-paths <file> and -H-paths <file>. This removes hardcoding libloc directory structure from compiler and makes it more similar to -I and -H files. We also remove libloc naming completely to make it simpler.

This change is not backwards compatible, but this is completely fine and expected. We don't use libloc anywhere in production yet.

[gretay-js]

Other executables such as the ones in extract_externals/extract_externals.ml and toplevel may need to be updated. Maybe search for all the place where include_dirs and hidden_include_dirs are used, if you haven't yet.