import-checker: ignore nested imports

contrib: add an import checker This checks for cycles in the module graph and verifies that imports of stdlib modules are not on the same line as relative imports of mercurial modules.

makememctx: move from patch to context to break import cycle

cleanup: move stdlib imports to their own import statement There are a few warnings still produced by my import checker, but those are false positives produced by modules that share a name with stdlib modules.

pathutil: tease out a new library to break an import cycle from canonpath use

subsettable: move from repoview to branchmap, the only place it's used This is a step towards breaking an import cycle between revset and repoview. Import cycles happened to work in Python 2 with implicit relative imports, but breaks on Python 3 when we start using explicit relative imports via 2to3 rewrite rules.