cmd/go/internal/modfile: prepare to move to x/mod

- Deleted dead code in gopkgin.go.
- Minor documentation changes.

Updates #34924

Change-Id: Ie2c744bbf6662cae20f09163200f20d7589fd237
Reviewed-on: https://go-review.googlesource.com/c/go/+/202565
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/modfile/gopkgin.go

@@ -1,47 +0,0 @@
-// Copyright 2018 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// TODO: Figure out what gopkg.in should do.
-
-package modfile
-
-import "strings"
-
-// ParseGopkgIn splits gopkg.in import paths into their constituent parts
-func ParseGopkgIn(path string) (root, repo, major, subdir string, ok bool) {
-	if !strings.HasPrefix(path, "gopkg.in/") {
-		return
-	}
-	f := strings.Split(path, "/")
-	if len(f) >= 2 {
-		if elem, v, ok := dotV(f[1]); ok {
-			root = strings.Join(f[:2], "/")
-			repo = "github.com/go-" + elem + "/" + elem
-			major = v
-			subdir = strings.Join(f[2:], "/")
-			return root, repo, major, subdir, true
-		}
-	}
-	if len(f) >= 3 {
-		if elem, v, ok := dotV(f[2]); ok {
-			root = strings.Join(f[:3], "/")
-			repo = "github.com/" + f[1] + "/" + elem
-			major = v
-			subdir = strings.Join(f[3:], "/")
-			return root, repo, major, subdir, true
-		}
-	}
-	return
-}
-
-func dotV(name string) (elem, v string, ok bool) {
-	i := len(name) - 1
-	for i >= 0 && '0' <= name[i] && name[i] <= '9' {
-		i--
-	}
-	if i <= 2 || i+1 >= len(name) || name[i-1] != '.' || name[i] != 'v' || name[i+1] == '0' && len(name) != i+2 {
-		return "", "", false
-	}
-	return name[:i-1], name[i:], true
-}

src/cmd/go/internal/modfile/print.go

@@ -12,6 +12,7 @@ import (
 	"strings"
 )
 
+// Format returns a go.mod file as a byte slice, formatted in standard style.
 func Format(f *FileSyntax) []byte {
 	pr := &printer{}
 	pr.file(f)

src/cmd/go/internal/modfile/read.go

@@ -17,7 +17,8 @@ import (
 	"unicode/utf8"
 )
 
-// A Position describes the position between two bytes of input.
+// A Position describes an arbitrary source position in a file, including the
+// file, line, column, and byte offset.
 type Position struct {
 	Line     int // line in input (starting at 1)
 	LineRune int // rune in line (starting at 1)