modfile: forward to x/mod (#208)

The canonical implementation of this code has now been made available
inside the x/mod module, so use that. The API is almost identical,
with the exception of `ParseGopkgIn` which isn't there.

go.mod

@@ -1,3 +1,5 @@
 module github.com/rogpeppe/go-internal
 
 go 1.19
+
+require golang.org/x/mod v0.9.0

go.sum

@@ -0,0 +1,2 @@
+golang.org/x/mod v0.9.0 h1:KENHtAZL2y3NLMYZeHY9DW8HW8V+kQyJsY/V9JlKvCs=
+golang.org/x/mod v0.9.0/go.mod h1:iBbtSCu2XBx23ZKBPSOrRkjjQPZFPuis4dIYUhu/chs=

modfile/forward.go

@@ -0,0 +1,57 @@
+// Package modfile implements parsing and formatting for go.mod files.
+//
+// This is now just a simple forwarding layer over golang.org/x/mod/modfile
+// apart from the ParseGopkgIn function which doesn't exist there.
+//
+// See that package for documentation.
+package modfile
+
+import (
+	"golang.org/x/mod/modfile"
+)
+
+type Position = modfile.Position
+type Expr = modfile.Expr
+type Comment = modfile.Comment
+type Comments = modfile.Comments
+type FileSyntax = modfile.FileSyntax
+type CommentBlock = modfile.CommentBlock
+type Line = modfile.Line
+type LineBlock = modfile.LineBlock
+type LParen = modfile.LParen
+type RParen = modfile.RParen
+type File = modfile.File
+type Module = modfile.Module
+type Go = modfile.Go
+type Require = modfile.Require
+type Exclude = modfile.Exclude
+type Replace = modfile.Replace
+type VersionFixer = modfile.VersionFixer
+
+func Format(f *FileSyntax) []byte {
+	return modfile.Format(f)
+}
+
+func ModulePath(mod []byte) string {
+	return modfile.ModulePath(mod)
+}
+
+func Parse(file string, data []byte, fix VersionFixer) (*File, error) {
+	return modfile.Parse(file, data, fix)
+}
+
+func ParseLax(file string, data []byte, fix VersionFixer) (*File, error) {
+	return modfile.ParseLax(file, data, fix)
+}
+
+func IsDirectoryPath(ns string) bool {
+	return modfile.IsDirectoryPath(ns)
+}
+
+func MustQuote(s string) bool {
+	return modfile.MustQuote(s)
+}
+
+func AutoQuote(s string) string {
+	return modfile.AutoQuote(s)
+}