Changes to flatpak-builder on master

[Alexander Larsson]

I just pushed a rework of the cache for flatpak-builder to master. It
now always builds with the install directory containing files that are
hardlinked into the cache. This means checking out, as well as checking
in is much faster. The first since its a hardlink instead of a copy,
and the second because we don't have to calculate sha256 checksum on
commit for the files that are already in the cache.

However, since the files are in the cache it is absolutely forbidden to
do in-place modifications of them, as it would break the cache ostree
repo. So, we use the rofiles-fuse filesystem to expose a copy of the
checkout where all hardlinked files are read-only. This required some
fixes in flatpak-builder where we did in-place editing, and it *may*
cause problems for third party json apps if the build of a module does
in-place modifications of files from another module. This is unlikely,
but *might* happen, so please test your json files.


Oh, and the change means all your caches are invalidated, so things
will re-build.

-- 
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
 Alexander Larsson                                            Red Hat, Inc 
       alexl at redhat.com            alexander.larsson at gmail.com 
He's a bookish pirate barbarian gone bad. She's a ditzy Buddhist 
magician's assistant with someone else's memories. They fight crime!