hook: add a 'hashook' function to test for hook existence Preparing the data for some hooks can be expensive. Add a function to check if a hook exists so we can skip useless preparation if no hook is configured.

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).