Ignore underscore-led files when parsing

Fixes sdboyer/gps#211.

_testdata/src/skip_/_a.go

@@ -0,0 +1,11 @@
+package skip
+
+import (
+	"bytes"
+	"sort"
+)
+
+var (
+	_ = sort.Strings
+	_ = bytes.Buffer
+)

_testdata/src/skip_/a.go

@@ -0,0 +1,12 @@
+package skip
+
+import (
+	"sort"
+
+	"github.com/sdboyer/gps"
+)
+
+var (
+	_ = sort.Strings
+	_ = gps.Solve
+)

pkgtree/pkgtree.go

@@ -195,6 +195,10 @@ func fillPackage(p *build.Package) error {
 	var testImports []string
 	var imports []string
 	for _, file := range gofiles {
+		// Skip underscore-led files, in keeping with the rest of the toolchain.
+		if filepath.Base(file)[0] == '_' {
+			continue
+		}
 		pf, err := parser.ParseFile(token.NewFileSet(), file, nil, parser.ImportsOnly|parser.ParseComments)
 		if err != nil {
 			if os.IsPermission(err) {

pkgtree/pkgtree_test.go

@@ -1153,6 +1153,26 @@ func TestListPackages(t *testing.T) {
 				},
 			},
 		},
+		"skip underscore": {
+			fileRoot:   j("skip_"),
+			importRoot: "skip_",
+			out: PackageTree{
+				ImportRoot: "skip_",
+				Packages: map[string]PackageOrErr{
+					"skip_": {
+						P: Package{
+							ImportPath:  "skip_",
+							CommentPath: "",
+							Name:        "skip",
+							Imports: []string{
+								"github.com/sdboyer/gps",
+								"sort",
+							},
+						},
+					},
+				},
+			},
+		},
 		// This case mostly exists for the PackageTree methods, but it does
 		// cover a bit of range
 		"varied": {