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Cache os calls to make xenv faster

Rosen Matev requested to merge rmatev/Gaudi:xenv-caching into master

When building the LHCb project, there are many repeated calls to os.listdir for the same directory. With a simple caching of the calls, I am able to save 275user + 350system. This is on a 4 core openstack VM.

Here are the timings of two builds with a hot ccache before and after the proposed change

1291.85user 773.61system 18:12.69elapsed 189%CPU (0avgtext+0avgdata 3290288maxresident)k
72941752inputs+5380936outputs (151956major+27552186minor)pagefaults 0swaps

1026.60user 424.04system 9:44.70elapsed 248%CPU (0avgtext+0avgdata 3290452maxresident)k
72332696inputs+5381144outputs (151712major+27911347minor)pagefaults 0swaps

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