check-code: factor out boot procedure into main This is a part of preparation for adding check-code.py extra checks by another python script in subsequent patch. This is also useful for SkeletonExtensionPlan. https://www.mercurial-scm.org/wiki/SkeletonExtensionPlan

perf: import newer modules separately for earlier Mercurial demandimport of early Mercurial loads an imported module immediately, if a module is imported absolutely by "from a import b" style. Recent perf.py satisfies this condition, because it does: - have "from __future__ import absolute_import" line - use "from a import b" style for modules in "mercurial" package Before this patch, importing modules below prevents perf.py from being loaded by earlier Mercurial, because these aren't available in such Mercurial, even though there are some code paths for Mercurial earlier than 1.9. - branchmap 2.5 (or bcee63733aad) - repoview 2.5 (or 3a6ddacb7198) - obsolete 2.3 (or ad0d6c2b3279) - scmutil 1.9 (or 8b252e826c68) For example, setting "_prereadsize" attribute in perfindex() and perfnodelookup() is effective only with Mercurial earlier than 1.8 (or 61c9bc3da402). After this patch, "mercurial.error" is the only blocker in "from mercurial import" statement for loading perf.py with Mercurial earlier than 1.2. This patch ignores it, because just importing it separately isn't enough.