cmd/go/internal/vet: print line numbers appropriately on list errors

Fixes golang#36173

For reasons that are unclear to me, this commit:
golang@f1d5ce0
introduces a TestPackagesFor function that strips line numbers from error
messages. This commit introduces a new version of that function for 'go vet'
that always keeps the line numbers.

src/cmd/go/internal/vet/vet.go

@@ -70,7 +70,7 @@ func runVet(cmd *base.Command, args []string) {
 
 	root := &work.Action{Mode: "go vet"}
 	for _, p := range pkgs {
-		_, ptest, pxtest, err := load.TestPackagesFor(p, nil)
+		_, ptest, pxtest, err := testPackagesFor(p)
 		if err != nil {
 			base.Errorf("%v", err)
 			continue
@@ -88,3 +88,34 @@ func runVet(cmd *base.Command, args []string) {
 	}
 	b.Do(root)
 }
+
+// testPackagesFor is like load.TestPackagesAndErrors but it returns
+// an error if the test packages or their dependencies have errors.
+// Only test packages without errors are returned.
+func testPackagesFor(p *load.Package) (pmain, ptest, pxtest *load.Package, err error) {
+	pmain, ptest, pxtest = load.TestPackagesAndErrors(p, nil)
+	for _, p1 := range []*load.Package{ptest, pxtest, pmain} {
+		if p1 == nil {
+			// pxtest may be nil
+			continue
+		}
+		if p1.Error != nil {
+			err = p1.Error
+			break
+		}
+		if len(p1.DepsErrors) > 0 {
+			err = p1.DepsErrors[0]
+			break
+		}
+	}
+	if pmain.Error != nil || len(pmain.DepsErrors) > 0 {
+		pmain = nil
+	}
+	if ptest.Error != nil || len(ptest.DepsErrors) > 0 {
+		ptest = nil
+	}
+	if pxtest != nil && (pxtest.Error != nil || len(pxtest.DepsErrors) > 0) {
+		pxtest = nil
+	}
+	return pmain, ptest, pxtest, err
+}

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

@@ -0,0 +1,19 @@
+env GO111MODULE=off
+
+# Issue 36173. Verify that "go vet" prints line numbers on load errors.
+
+! go vet a/a_test.go
+stderr 'a_test.go:4:3: use of internal package'
+
+-- a/a.go --
+package a
+
+-- a/a_test.go --
+package a
+
+import (
+  _ "a/x/internal/y"
+)
+
+-- a/x/internal/y/y.go --
+package y