This moves all the files from lib into common, and it also adds all the libflatpak
sources into libflatpak-common, making libflatpak just a wrapper around the common
helper library.
This move allows the CLI to use all the code from libflatpak. We were already doing
this with a few things like flatpak-error*.[ch], and we want to do it even more
when sharing FlatpakTransaction. This also allows use to slowly move
the CLI to using the libflatpak apis for some things.
Closes: #1706
Approved by: alexlarsson
