Handle both henrietta-cache cache formats

Handle both egg source directory layouts: with or without the "tags"
directory.
Revert "Bump version to 1.0.0"

This reverts commit 59bb35eb5d59e7303a3672cdf8351608cc04ce82.

No need to bump the major version, since egg-tarballs handles both
henrietta-cache cache formats.
Bump version to 0.5.0