Extract clearing of import string cache to private method
Extract clearing of import string cache to private method

The fact that we had added a comment every time we did this is a bit of
a code smell so I decided to extract this bit to a well-named method.

trotzig/import-js,trotzig/import-js,trotzig/import-js,lencioni/import-js,lencioni/import-js,Galooshi/import-js,lencioni/import-js