Enforce import grouping

This is a new hacking rule (H305) that enforces the hacking
guideline about grouping stdlib, third-party, and project
imports together.  It assumes any imports that are not found are
third party so the tool can be run in environments without all of
a project's deps installed.  This should be safe because stdlib
and project imports should always be available.

Change-Id: I1cfa85745e4da3e773296d623ba390c5edd11ef5
Enforce grouping like imports together

This rule (H307) enforces that all imports from a given type are
kept together.  Separating them with unnecessary whitespace goes
against the import guidelines and breaks our alphabetical order
checking.

Change-Id: I80721599a63e2f323d17d25822a167e44f566164
Merge "Enforce import grouping"
Merge "Enforce grouping like imports together"