setup: properly install build_hgextindex for py2exe builds
Because the hgbuild class has a private copy of build.sub_commands,
modifying build.sub_commands from this code effectively resulted
in a no-op. Registering the sub-command on hgbuild actually results
in the sub-command running when building Mercurial.
Differential Revision: https://phab.mercurial-scm.org/D6093
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273ce12ad8f1` will report some style violations on a very old commit).