.arcconfig

requirements: introduce a set of working directory specific requirements

Some requirements like the SPARSE_REQUIREMENT is working directory specific and
cannot be shared. We add a set which will contain all these requirements.

This is not the best we can do, I think having a rich requirement class will be
much better but that will be out of scope for this series.

Differential Revision: https://phab.mercurial-scm.org/D8924

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