importer produces dupes in the CSS when the pantry package is npm linked into the project

[elyseholladay]

To reproduce the error:

cd retailmenot
rm -rf node_modules
npm install
grunt fresh
# note that css like button mixin is not duplicated in the built css in www/gui/build/webpack/desktop
npm link ../roux-ingredients
grunt fresh
# note that css like button mixin is duplicated in the built css in www/gui/build/webpack/desktop

@lzilioli: "I suspect that this was caused by the fact that if pantry instanceOf Pantry, we exited early and did not apply the dedupe logic. I switched this to Pantry.isPantry here which I think will resolve it. I'll close this out once I confirm."

Issue still exists for non-entrypoint sass files.