Always add Plural metadata to dummy translations.

When this was conditional, we'd have plural info in some files, not in
others. Then we'd merge them, and msgcat would add wacky markers to
indicate the difference, and the metadata wouldn't parse.
Tighten up the extraction I/O

We were writing the main file twice.  No need.
Merge pull request #2303 from edx/ned/i18n-tool-fixes

Ned/i18n tool fixes