reorganize egg repository to cut down working copy size

[felix winkelmann]

Checking out the complete egg repository requires a considerable amount of disk space. During a discussion with ashinn, sjamaan, iraikov, zbigniew and mario, we came to the conclusion that it would be a win if one could merely check out the development sources, not the tagged copies. One possible new layout would be:

stable/release/4/spiffy.scm
stable/release/4/spiffy.setup
...
dev/release/4/spiffy/1.0
dev/release/4/spiffy/1.1
...

Things to keep in mind:

• the henrietta instances have to be updated to export from the correct URLs
• the restructuring will invalidate all working copies and likely make it very difficult to check in uncommitted changes, so the change should be announced early enough and be done during of period in which no other repo access can take place