cmd/go: diagnose missing replacement directories

I noticed the missing diagnostic when writing a regression test for #33795.

Change-Id: Ic3249436a6109d71f9ff720b7096f9b872f6a94b
Reviewed-on: https://go-review.googlesource.com/c/go/+/212201
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/modload/load.go

@@ -1320,6 +1320,21 @@ func fetch(mod module.Version) (dir string, isLocal bool, err error) {
 			if !filepath.IsAbs(dir) {
 				dir = filepath.Join(ModRoot(), dir)
 			}
+			// Ensure that the replacement directory actually exists:
+			// dirInModule does not report errors for missing modules,
+			// so if we don't report the error now, later failures will be
+			// very mysterious.
+			if _, err := os.Stat(dir); err != nil {
+				if os.IsNotExist(err) {
+					// Semantically the module version itself “exists” — we just don't
+					// have its source code. Remove the equivalence to os.ErrNotExist,
+					// and make the message more concise while we're at it.
+					err = fmt.Errorf("replacement directory %s does not exist", r.Path)
+				} else {
+					err = fmt.Errorf("replacement directory %s: %w", r.Path, err)
+				}
+				return dir, true, module.VersionError(mod, err)
+			}
 			return dir, true, nil
 		}
 		mod = r

src/cmd/go/testdata/script/mod_replace_import.txt

@@ -28,7 +28,8 @@ stdout 'example.com/v v1.12.0 => ./v12'
 cd fail
 ! go list all
 stdout 'localhost.fail'
-stderr '^can.t load package: m.go:3:8: module w@latest found \(v0.0.0-00010101000000-000000000000, replaced by ../w\), but does not contain package w$'
+stderr '^can''t load package: m.go:4:2: module w@latest found \(v0.0.0-00010101000000-000000000000, replaced by ../w\), but does not contain package w$'
+stderr '^can''t load package: m.go:5:2: nonexist@v0.1.0: replacement directory ../nonexist does not exist$'
 
 -- go.mod --
 module example.com/m
@@ -128,7 +129,10 @@ package i
 -- fail/m.go --
 package main
 
-import _ "w"
+import (
+	_ "w"
+	_ "nonexist"
+)
 
 func main() {}
 
@@ -137,3 +141,4 @@ module localhost.fail
 
 replace w => ../w
 
+replace nonexist v0.1.0 => ../nonexist