hg: update top-level script to use modern import conventions

simplemerge: update to conform with modern import conventions

contrib: inform import checker that __builtin__ is a thing

contrib: allow importing "symbols" from mercurial

hgperf: update to conform with import style checks

contrib: update hg-ssh to conform with import style checks

contrib: have import-checker work mostly with native strings for mod names Module names are a bit awkward to deal with portably otherwise.

bundlerepo: move bundle2 part handling out to a function This moves the bundle2 part handling for bundlerepo out to a separate function so extensions can participate in bundlerepo setup when using bundle2 bundles. Differential Revision: https://phab.mercurial-scm.org/D290

bundle2: seek part back during iteration Previously, iterparts would yield the part to users, then consume the part. This changed the part after the user was given it and left it at the end, both of which seem unexpected. Let's seek back to the beginning after we've consumed it. I tried not seeking to the end at all, but that seems important for the overall bundle2 consumption. This is used in a future patch to let us move the bundlerepo bundle2-changegroup-part to be handled entirely within the for loop, instead of having to do a seek back to 0 after the entire loop finishes. Differential Revision: https://phab.mercurial-scm.org/D289

bundlerepo: move temp bundle creation to a separate function A future patch will refactor certain parts of bundlerepo initiatlization such that we need to create temp bundles from another function. Let's move this to another function to support that. Differential Revision: https://phab.mercurial-scm.org/D288