#490 moves import lines to the top of the stanza, but it only happens the first time hpack is called.

Repro:

1. Run rm *.cabal
2. Add verbatim: { import: ... } line to a stanza
3. Run hpack
4. Cabal file incorrectly has import line at bottom
5. Run hpack again
6. Cabal file correctly has import line moved to top