Export org.fedoraproject.p2.installer package
[test] Implement first XMvn installer test
[test] Implement buildroot expectation assertions

Tests can assert certain structure of resulting buildroot (presence of
plugins, features or symlinks in given dropin).  Test will fail if
there is any unexpected item or if any expected item is missing.
Fix installer state leak bug

All installer data structures have to be reinitialized on each
installation request.  This bug didn't affect package builds as XMvn
calls installer only once, but prevented unit testing from working as
expected.
[test] InstallerTest refactoring
[test] Implement more installer tests

This adds test for subpackage split, dependency resolution and
symlinking dependencies.