perf: rearrange imports of changelong and manifest to appease check-code
convert: move import of xml.minidom.dom to its own line for check-code
subrepo: move import of xml.minidom.dom to its own line for check-code
patch: correct import of email module
mail: correct import of email module
patchbomb: correct import of email module
notify: correct import of email module, sort stdlib modules to top
check-code: new rule to forbid imports of a.b on the same line as other imports

This style of import can trip up 2to3 and cause it to produce invalid
files if one of the imports is supposed to be a relative import. This
prevents that behavior, and in the process exposed a lot of silly
import errors related to the email module.