.arcconfig

demandimport: make module ignores a set (API)

The list of modules to ignore is used for membership testing. Yet
it is defined as a list.

Sets are more efficient for membership testing. So this commit converts
the module list to a set.

Since we took an API hit, I renamed the variable to further clarify
the change.

This appears to reduce the CPU time for running 300 invocations of
`hg log -r. -T '{rev}'` on my i7-6700K:

before: 18.64s
after:  18.44s

Differential Revision: https://phab.mercurial-scm.org/D3440

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