llgo: Custom package importer/exporter. Fixes issue #43

I suggest making the drone.io builder run llgo-dist through twice.
The first time it'll use the "regular" gc importer for uncompiled
packages. The second run, all packages have been compiled and should
thus use the custom importer.
Merge pull request #89 from quarnster/exports

llgo: Custom package importer/exporter. Fixes issue #43