Ensures we get a canonical tag per langtags.json as far as we possibly
can. This is a breaking change for the compiler as tags which were
formerly regarded as canonical are no longer regarded that way. This
mostly relates to script subtag but a secondary bug meant that some
other tags would have lost data in the canonicalization process
(because we did a lookup based only on the language subtag previously,
which is a no-no).
See keymanapp/keyboards#1452 for related work.
