make evaluation of spec+cache independent of build pass context

make evaluation of spec+cache independent of build pass context

don't inject the build pass specific variables into the project even
before evaluating the .spec file and the .qmake.cache. they are not
supposed to base configuration on that - feature files should do that
later.

the immediate advantage of this is that base_vars is never manipulated
upfront any more, which allows for cleaner setup paths. also, we can do
more caching of the spec+cache contents.

Change-Id: I19d7f8bec1fb7c3b54121e26794340b287055ebf
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>