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

@@ -55,7 +55,15 @@ func TestPackagesFor(p *Package, cover *TestCover) (pmain, ptest, pxtest *Packag
 		}
 		if len(p1.DepsErrors) > 0 {
 			perr := p1.DepsErrors[0]
-			perr.Pos = "" // show full import stack
+			// A typical ImportStack looks like
+			// [ "command-line-arguments", "(test)", "real/pkg/name" ]
+			// so we want to look at index 2 to see if the error is in the package
+			// we're looking at directly.
+			direct := len(perr.ImportStack) <= 2 ||
+				len(perr.ImportStack) == 3 && perr.ImportStack[2] == p1.ImportPath
+			if !direct {
+				perr.Pos = "" // show full import stack
+			}
 			err = perr
 			break
 		}

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