[REFACTOR|FIX] Refactors IDEA project.fs package

Refactors some structural issues, fixes several bugs
related to file paths. ProjectImp will be refactored in a
later patch, to not cache resources any more, but to use io.File.

Change-Id: If752256bff661ec5954df36a22f5952e3cff0002
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1963
Tested-by: Jenkins CI
Reviewed-by: Holger Schmeisky <google@schmeisky.com>