Importer should import a single version in each run

[James Westby]

In order to reduce the critical section of the importer code, when import_package.py is run it should import a single
version of the package at a time.

It could either do this by importing the first version, then exiting, signalling to the parent that it should immediately
be re-run.

Otherwise it could do the loop internally with some code shuffling.

The latter would be runtime cheaper.

Thanks,

James