OPT: Do not decorate imports of already processed or queued up modules
OPT: Do not decorate imports of already processed or queued up modules

This cuts time penalty a few times, e.g. mvpa2.suite now imports only in 4.20 (vs stock 2.40s)
sec, thus under 100% penalty altogether

mvdoc/duecredit,jgors/duecredit