modfile: improve directory path detection and error text consistency

An error text suggests a directory path needs to start with ./ or ../
if it's a relative path, but in reality relative paths with .\ and ..\
prefix (such as those that are used on Windows) are also accepted.

Furthermore, a relative path like ./ or ../ is fine, as are ./. and
../., but the cleaner and shorter equivalent relative paths . and ..
are reported as if they're not directory paths (even though a module
path cannot consist of nothing but dots).

Fix those inconsistencies and make IsDirectoryPath report true on "."
and ".." paths as expected, and make its documentation clear that
a path like "sub/dir", despite being a relative path, is interpreted
as a module path.

For golang/go#60572.

Change-Id: I8fa4a2c66bc83a1ccafc453b96f3bb33dc222cd1
Reviewed-on: https://go-review.googlesource.com/c/mod/+/500335
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

modfile/rule.go

@@ -542,7 +542,7 @@ func parseReplace(filename string, line *Line, verb string, args []string, fix V
 			if strings.Contains(ns, "@") {
 				return nil, errorf("replacement module must match format 'path version', not 'path@version'")
 			}
-			return nil, errorf("replacement module without version must be directory path (rooted or starting with ./ or ../)")
+			return nil, errorf("replacement module without version must be directory path (rooted or starting with . or ..)")
 		}
 		if filepath.Separator == '/' && strings.Contains(ns, `\`) {
 			return nil, errorf("replacement directory appears to be Windows path (on a non-windows system)")
@@ -555,7 +555,6 @@ func parseReplace(filename string, line *Line, verb string, args []string, fix V
 		}
 		if IsDirectoryPath(ns) {
 			return nil, errorf("replacement module directory path %q cannot have version", ns)
-
 		}
 	}
 	return &Replace{
@@ -679,14 +678,15 @@ func (f *WorkFile) add(errs *ErrorList, line *Line, verb string, args []string,
 	}
 }
 
-// IsDirectoryPath reports whether the given path should be interpreted
-// as a directory path. Just like on the go command line, relative paths
+// IsDirectoryPath reports whether the given path should be interpreted as a directory path.
+// Just like on the go command line, relative paths starting with a '.' or '..' path component
 // and rooted paths are directory paths; the rest are module paths.
 func IsDirectoryPath(ns string) bool {
 	// Because go.mod files can move from one system to another,
 	// we check all known path syntaxes, both Unix and Windows.
-	return strings.HasPrefix(ns, "./") || strings.HasPrefix(ns, "../") || strings.HasPrefix(ns, "/") ||
-		strings.HasPrefix(ns, `.\`) || strings.HasPrefix(ns, `..\`) || strings.HasPrefix(ns, `\`) ||
+	return ns == "." || strings.HasPrefix(ns, "./") || strings.HasPrefix(ns, `.\`) ||
+		ns == ".." || strings.HasPrefix(ns, "../") || strings.HasPrefix(ns, `..\`) ||
+		strings.HasPrefix(ns, "/") || strings.HasPrefix(ns, `\`) ||
 		len(ns) >= 2 && ('A' <= ns[0] && ns[0] <= 'Z' || 'a' <= ns[0] && ns[0] <= 'z') && ns[1] == ':'
 }