sack: Also look in /usr/share/rpm for Packages

[cgwalters]

In the rpm-ostree model, the RPM database lives in /usr/share/rpm, and
is immutable. Let's look there first, and then fall back to the old
mutable database.

[cgwalters]

See also openSUSE/libsolv#43

[akozumpl]

Thank you for the patch, Colin, we're of course ready to help with OSTree support.

The patch in this pull request however looks like it wasn't tested properly and even introduces a regression:

>>> import hawkey
>>> s = hawkey.Sack(make_cache_dir=True)
>>> s.load_system_repo()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IOError: load_system_repo() failed.
>>> 

Works on master. From the patch it's straightforward to see why---if the OSTree's rpmdb location is not available we only assign something to a local variable and error out in current_rpmdb_checksum().

The proper fix should introduce different rpm locations as a null terminated array (needs to be moved out of header files) and a loop that goes through them too.

[cgwalters]

Sure; do you want me to remove the HY_SYSTEM_RPMDB define? It feels like the array should be private, right?

[akozumpl]

Yeah, it should be private.

Fine by you and PK, @hughsie ?

[hughsie]

Fine by me, thanks.