Loader: support dirs and subdirs in .autoload and support ^ in matches

[hhaensel]

Currently, .autoload only supports files in the root_dir.

This PR refactors sort_load_order() and autoload() to support

• any file within the root_dir including subdirs (any pathseparator is allowed)
• dirs and subdirs
• ^ to mean the beginning of filenames relative to root_dir for matches (namematch, skipmatch)

proposed ToDos:

• expand namematch and skipmatch to directories
• find a notation to include/exclude regex file patterns, e.g.
  • --^skip/a.* for excluding files
  • ++^noskip/a.* for including files

@essenciary @PGimenez what do you think?

Alternatively we could implement Glob.jl-matching, which sounds tempting.

[essenciary]

@hhaensel Great idea, thank you! I'm OK with Glob.jl, I see that it has no dependencies and is actively maintained.

[hhaensel]

There's an open issue in Glob.jl to add globstar support ("**/" syntax).
I just added a respective PR, let's hope that it passes all the edge cases. It would strongly simplify the docs, because people know that notation from .gitignore.