internal/fetch: remove empty-module-path code
internal/fetch: remove empty-module-path code

An empty module path in fetch won't happen since we made the
DirectoryModuleGetter figure out its module path.

For golang/go#47834

Change-Id: I953ad425c7e13ec4e771f4160870e5ef8f7349ff
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/343960
Trust: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite