cmd/go: 'go get […]/v2@v2.X.Y' fails when the go.mod file for […]/v2 is at the repo root [1.13 backport]

[gopherbot]

@bcmills requested issue #34383 to be considered for backport to the next 1.13 minor release.

@gopherbot, please backport to 1.13. This is a regression, and the workaround (editing the go.mod file instead of using go get) does not work for downgrading.

[gopherbot]

Change https://golang.org/cl/197063 mentions this issue: [release-branch.go1.13] cmd/go: suppress errors in package-to-module queries if the package is already found

[gopherbot]

Closed by merging 1dc1e7c to release-branch.go1.13.