modules: tests that ensure that from the same git commit (clean or otherwise) that we get identical module output

[myitcv]

Determining that a module's contents correctly corresponds to its declared git commit is one of the reasons
we added the source field to module.cue. We should have tests that check that.

One challenge is that creating a module involves compressing the contents with the zip deflate compressor,
but there's no guarantee that that's deterministic over time and changing Go versions. This means
it might be better to check the contents of the zip file and the module manifest rather than requiring
a specific checksum for the manifest and its contents.