godoc: add 'shallow' PageInfoMode to only show top-level directories in subdirectory listings

[seeruk]

Solves golang/go#27666. I'm not sure if this is the preferred method of implementing something like this, but it does the job!

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[gopherbot]

Message from Gerrit User 5065:

Patch Set 1:

I think we're waiting on a decision on the GitHub issue first to see what we want to do.

I'm going to wait for a decision there before reviewing code.

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