Implicit dependency scanning takes significant time and, since it doesn't
produce files, it's okay to keep the resulting information across clean
commands as long as the scanner is triggered again if there's need to. This
commit accomplishes that.
The advantage of this approach can be observed by the following timings when
building the group "bin":
Method Time
------------------------------------------------------------------------
Fully clean build 5m18.633s
Clean build with scanning result persisted 4m23.346s
Clean build with ccache but non-persistent scan results 1m40.125s
Clean build with scanning results persisted and with ccache 14.843s
While at it, move management of information persisted across clean commands to
a separate module.
Gustavo Jose de Sousa