[release-branch.go1.13] cmd/go: suppress more errors in package-to-module loading

In CL 197059, I suppressed errors if the target package was already found.
However, that does not cover the case of passing a '/v2' module path to
'go get' when the module does not contain a package at its root.

This CL is a minimal fix for that case, intended to be backportable to 1.13.

(Longer term, I intend to rework the version-validation check to treat
all mismatched paths as ErrNotExist.)

Updates #34746
Fixes #34747

Change-Id: Ia963c2ea00fae424812b8f46a4d6c2c668252147
Reviewed-on: https://go-review.googlesource.com/c/go/+/199839
Run-TryBot: Bryan C. Mills <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Jay Conrod <[email protected]>
(cherry picked from commit 421d35c)
Reviewed-on: https://go-review.googlesource.com/c/go/+/199997

Author: Bryan C. Mills

src/cmd/go/internal/modload/query.go

@@ -472,10 +472,10 @@ func queryPrefixModules(candidateModules []string, queryModule func(path string)
 					notExistErr = rErr
 				}
 			} else if err == nil {
-				if len(found) > 0 {
-					// golang.org/issue/34094: If we have already found a module
-					// containing the target package, ignore errors for modules with
-					// shorter paths.
+				if len(found) > 0 || noPackage != nil {
+					// golang.org/issue/34094: If we have already found a module that
+					// could potentially contain the target package, ignore unclassified
+					// errors for modules with shorter paths.
 
 					// golang.org/issue/34383 is a special case of this: if we have
 					// already found example.com/foo/[email protected] with a matching go.mod

src/cmd/go/testdata/script/mod_get_major.txt

@@ -13,6 +13,11 @@ go get -d vcs-test.golang.org/git/v3pkg.git/[email protected]
 go list -m vcs-test.golang.org/git/v3pkg.git/v3
 stdout '^vcs-test.golang.org/git/v3pkg.git/v3 v3.0.0$'
 
+go get -d vcs-test.golang.org/git/empty-v2-without-v1.git/[email protected]
+
+go list -m vcs-test.golang.org/git/empty-v2-without-v1.git/v2
+stdout '^vcs-test.golang.org/git/empty-v2-without-v1.git/v2 v2.0.0$'
+
 -- go.mod --
 module example.com
 go 1.13