refs #13245/#14668 - use builddir consistently and verify usage of cached results in selfcheck

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discarding cached result from 'b1/main.a1' for 'cli/main.cpp' - hash mismatch (16146913026568202460 / 18076949867239869788)
discarding cached result from 'b1/main.a1' for 'gui/main.cpp' - hash mismatch (18076949867239869788 / 16146913026568202460)

The problem is that we perform individual analysis(eses) on the same builddir leading to conflicts when a file with the same name exists in a different location.

I am not sure this can be resolved as the files.txt is written unconditionally essentially discarding.

A valid use case would be the usage of a cache in conjunction with an IDE integration which would only invoke the analysis for a single file. In that case though we could generate the files.txt for the whole project/inputs regardless of the selected subset. But that would only work with a fixed project configuration but not in the case of this PR when you extract configurations.

It could be achieved though by not writing the file unconditionally but use it as an index for lookups and append new entries. This would leave outdated and/or removed files behind but that is already an issue now (needs a separate ticket).

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discarding cached result from 'b1/main.a1' for 'cli/main.cpp' - hash mismatch (16146913026568202460 / 18076949867239869788)
discarding cached result from 'b1/main.a1' for 'gui/main.cpp' - hash mismatch (18076949867239869788 / 16146913026568202460)

The problem is that we perform individual analysis(eses) on the same builddir leading to conflicts when a file with the same name exists in a different location.

Reducing the file generated in the build dir down to the basename only seems to go a bit too far especially if you might have a directory structure with the same filenames. Maybe the (relative) directory should also be encoded into it. That way the cache should be usable if you pass into a project but only filter for a subset after a full analysis.