(SplitPackageBinding)

More simplifications:
- MB.getTopLevelPackage already handles the LE package cache, so
directly forward to it from LE.getToplevelPackage
- Reduce places where LE.TheNotFoundPackage is returned and handled
- More usages of PlainPackageBinding
- MB.getVisiblePackage already does addPackage, remove in MB.getPackage

Also adds a fix to make sure that ReconcilerTests9.testBug547113 doesn't
fail pre java 9

Change-Id: Ie28c61b531bc0992f047a634bbd856f9a79c9f62