This adds a source type that copies an entire directory, optionally skipping
some files. Additionally it also skips the .flatpak-directory dir and
the build dir to avoid weird recursion.
Since we can't really checksum an entire directory a dir source is always
rebuilt.
Closes: #136
Approved by: alexlarsson
All this does is construct a finish arg, but it makes it a lot nicer
to create extra-data using manifest. Additionally, it allows you to
create per-arch extra data if you set only-arches on the source.
Closes: #40
Approved by: alexlarsson