[Qbs] Why was protobuf.outputDir removed

[Oswald Buddenhagen]

On Tue, Sep 15, 2020 at 09:00:50AM +0200, Kai Dohmen via Qbs wrote:
>Furthermore when installing the generated header files I cannot use the
>Groups property installSourceBase to match the installed directory
>structure to the proto files ones.
>
that's actually an interesting one. the general idea is to get rid of 
accessible output dirs to make the build dirs as opaque as possible.  
however, this obviously defies installSourceBase usage with generated 
files (the original use case was to install trees of data files that are 
not generated). i think the way out would be finding the *sources* of 
the installed artifacts (if necessary, recursively), as their layout is 
actually controllable by the user, and consequently expect the base to 
point into the source tree. this shall fail if an artifact has multiple 
primary sources coming from different directories.