packaging: extract pre-computed version when running from plain tarball
The tarballs created by setup.py are not including the usual archive
markers that `hg archive` leaves, so the rewrite of the version number
computation actually left the empty version string around. This meant
that installations from PyPI would use 0.0.0 as version string.
Differential Revision: https://phab.mercurial-scm.org/D10502
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