Handle malformed go source case in ListPackages()

Prior to this, encountering malformed Go source code in any package
woudl cause the entire ListPackages operation to fail. Now, when the Go
source text scanner encounters an error, we catch it and store it
correctly in a PackageOrErr.

This may well not be exhaustive - we may need to cover other types.
Handles one aspect of sdboyer/gps#99.

_testdata/src/bad/bad.go

@@ -0,0 +1,2 @@
+// This ill-formed Go source file is here to ensure the tool is robust
+// against bad packages in the workspace.

analysis.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"fmt"
 	"go/build"
+	gscan "go/scanner"
 	"io"
 	"io/ioutil"
 	"os"
@@ -165,6 +166,12 @@ func ListPackages(fileRoot, importRoot string) (PackageTree, error) {
 			pkg = happy(ip, p)
 		} else {
 			switch terr := err.(type) {
+			case gscan.ErrorList, *gscan.Error:
+				// This happens if we encounter malformed Go source code
+				ptree.Packages[ip] = PackageOrErr{
+					Err: err,
+				}
+				return nil
 			case *build.NoGoError:
 				ptree.Packages[ip] = PackageOrErr{
 					Err: err,

analysis_test.go

@@ -3,6 +3,8 @@ package gps
 import (
 	"fmt"
 	"go/build"
+	"go/scanner"
+	"go/token"
 	"os"
 	"path/filepath"
 	"reflect"
@@ -225,8 +227,8 @@ func TestWorkmapToReach(t *testing.T) {
 
 func TestListPackages(t *testing.T) {
 	srcdir := filepath.Join(getwd(t), "_testdata", "src")
-	j := func(s string) string {
-		return filepath.Join(srcdir, s)
+	j := func(s ...string) string {
+		return filepath.Join(srcdir, filepath.Join(s...))
 	}
 
 	table := map[string]struct {
@@ -458,6 +460,28 @@ func TestListPackages(t *testing.T) {
 				},
 			},
 		},
+		"malformed go file": {
+			fileRoot:   j("bad"),
+			importRoot: "bad",
+			out: PackageTree{
+				ImportRoot: "bad",
+				Packages: map[string]PackageOrErr{
+					"bad": {
+						Err: scanner.ErrorList{
+							&scanner.Error{
+								Pos: token.Position{
+									Filename: j("bad", "bad.go"),
+									Offset:   113,
+									Line:     2,
+									Column:   43,
+								},
+								Msg: "expected 'package', found 'EOF'",
+							},
+						},
+					},
+				},
+			},
+		},
 		"two nested under empty root": {
 			fileRoot:   j("ren"),
 			importRoot: "ren",