QbsProjectManager: Do not assume "lib" directory name for external qbs.

While it is technically not required that project.libDirName matches the
external library directory name, other combinations seem quite silly. In
practice, they should either both be "lib" or both be "lib64".

Change-Id: Ib8ecba846787f7791a0b6bd57cdfc2df024dbb3b
Task-number: QBS-627
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>