cmd/go: 'go get […]/v2@v2.X.Y' fails when the repo root contains the go.mod file for […]/v2 but no .go source files [1.13 backport]

[gopherbot]

@bcmills requested issue #34746 to be considered for backport to the next 1.13 minor release.

@gopherbot, please backport to 1.13. This was a regression, and the fix goes with #34497 (already backported).

[gopherbot]

Change https://golang.org/cl/199997 mentions this issue: [release-branch.go1.13] cmd/go: suppress more errors in package-to-module loading

[gopherbot]

Closed by merging 842a1e7 to release-branch.go1.13.