build: for the bootstrap phase of a deb/rpm build pure-py mercurial is enough
When bootstrapping a deb/rpm build, packagelib.sh starts performing a local
build for the sole purpose of parsing the output of "hg version".
Then it "hg archive"s the source code, and builds everything again.
For that initial step, we are perfectly good in using a pure python mercurial,
without compiling the c modules (base85, bdiff, zstdlib, ...).
On my personal system, this cuts down 22 seconds for a package build (the
bootstrapping build goes from ~30 to ~8 seconds).
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