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

[bcmills]

My fix for #34383 was incomplete: it failed to take into account the possibility that the longest path for the module would be a *PackageNotInModuleError, in which case we should prefer the *PackageNotInModuleError over an unclassified error from a module with a shorter path.

A reproducer:

example.com$ gotip version
go version devel +cfe23204 Sun Oct 6 18:18:50 2019 +0000 linux/amd64

example.com$ GOPROXY=direct gotip get vcs-test.golang.org/git/empty-v2-without-v1.git/v2@v2.0.0
go: finding vcs-test.golang.org/git/empty-v2-without-v1.git v2.0.0
go get vcs-test.golang.org/git/empty-v2-without-v1.git/v2@v2.0.0: vcs-test.golang.org/git/empty-v2-without-v1.git@v2.0.0: invalid version: module contains a go.mod file, so major version must be compatible: should be v0 or v1, not v2

CC @jayconrod

[bcmills]

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

[gopherbot]

Backport issue(s) opened: #34747 (for 1.13).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://golang.org/wiki/MinorReleases.

[gopherbot]

Change https://golang.org/cl/199839 mentions this issue: cmd/go: suppress more errors in package-to-module loading

[gopherbot]

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