Rewrite command respects -i18n-strings-filename flag

This can be used to tell the tool exactly which strings you want to
translate. In general, you'll have a lot that shouldn't be.

The format of this is the foo.go.en.json format that -extract-strings
will provide.

[#71156874]
Ignore testfiles in -rewrite-package

[Finishes #71156874]
Move all the test fixtures

Once you find that there is a test fixture that is itself a test,  you probably
want to move them out of the directory where all your tests are at. You know.

It happens.