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/load/test.go

@@ -54,9 +54,7 @@ func TestPackagesFor(p *Package, cover *TestCover) (pmain, ptest, pxtest *Packag
 			break
 		}
 		if len(p1.DepsErrors) > 0 {
-			perr := p1.DepsErrors[0]
-			perr.Pos = "" // show full import stack
-			err = perr
+			err = p1.DepsErrors[0]
 			break
 		}
 	}

src/cmd/go/internal/test/test.go

@@ -728,12 +728,16 @@ func runTest(cmd *base.Command, args []string) {
 
 		buildTest, runTest, printTest, err := builderTest(&b, p)
 		if err != nil {
-			str := err.Error()
-			str = strings.TrimPrefix(str, "\n")
 			if p.ImportPath != "" {
-				base.Errorf("# %s\n%s", p.ImportPath, str)
+				// Strip off line number info so we can show the full import stack.
+				perr, ok := err.(*load.PackageError)
+				if ok {
+					perr.Pos = ""
+				}
+
+				base.Errorf("# %s\n%s", p.ImportPath, strings.TrimPrefix(err.Error(), "\n"))
 			} else {
-				base.Errorf("%s", str)
+				base.Errorf("%s", strings.TrimPrefix(err.Error(), "\n"))
 			}
 			fmt.Printf("FAIL\t%s [setup failed]\n", p.ImportPath)
 			continue

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