common.hpp: mark as implementation detail

refs #2098

Change-Id: Ibe8690eab5663be961b503d57932d5a59a57b54c
wscript: don't install detail/*.hpp

This commit also fixes include paths in face-impl.hpp.

refs #2266

Change-Id: I1bbca6cdb6ffaab134f52a383fe3efe36106244f