A main package can be imported in one edge case we weren't testing:
when the same main package has any tests, which implicitly import main.
Before the fix, we would panic:
> garble test -v ./...
# test/bar/somemaintest.test
panic: main packages should never need to obfuscate their import paths
Following the best practices from upstream.
In particular, the "txt" extension is somewhat ambiguous.
This may cause some conflicts due to the git diff noise,
but hopefully we won't ever do this again.