the cumulative evaluation has a good chance to make a mess of the
virtual file contents created by the exact parsing, so better contain it
to its own namespace.

the ProFile cache also needs to keep the files separate. this
specifically addresses the side issue discussed in QTCREATORBUG-10779.
it also fixes attempts to deploy the wrong build when the variant is
selected through a cache file, as in QTCREATORBUG-15815.

in the project explorer, we don't track from which evaluation pass
particular files came from, so we try the cumulative first to get the
most contents, and fall back to the exact one if the former file is
empty (or does not exist at all).

Task-number: QTCREATORBUG-15815
Change-Id: I2c1eb16c97526fa275a1c6a2eae9266d385859ac
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>