cmd/go: report original module path in error parsing replaced go.mod

MVS reports an error when a go.mod file declares a module path that
doesn't match the path it was required with. If the module is a
replacement, its declared path may be the original path (preferred) or
the replacement path.

This CL makes the reported error a little more clear: the "required as"
path should be the original required path, not the replacement path.

Fixes #38220

Change-Id: I08b50a100679a447c8803cca1d1b32bc115ec1b1
Reviewed-on: https://go-review.googlesource.com/c/go/+/227097
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modload/mvs.go

@@ -148,7 +148,7 @@ func (r *mvsReqs) required(mod module.Version) ([]module.Version, error) {
 	if mpath := f.Module.Mod.Path; mpath != origPath && mpath != mod.Path {
 		return nil, module.VersionError(mod, fmt.Errorf(`parsing go.mod:
 	module declares its path as: %s
-	        but was required as: %s`, mpath, mod.Path))
+	        but was required as: %s`, mpath, origPath))
 	}
 	if f.Go != nil {
 		r.versions.LoadOrStore(mod, f.Go.Version)

src/cmd/go/testdata/mod/example.com_quote_v1.5.2.txt

@@ -0,0 +1,9 @@
+This module is a replacement for rsc.io/quote, but its go.mod file declares
+a module path different from its location and the original module.
+
+-- .mod --
+module rsc.io/Quote
+
+go 1.14
+-- .info --
+{"Version":"v1.5.2"}

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

@@ -0,0 +1,23 @@
+# If a replacement module declares a module path different from both
+# the original module and its location, report an error with all three paths.
+# In particular, the "required as" path should be the original.
+# Verifies golang.org/issue/38220.
+! go list .
+cmp stderr want
+
+-- go.mod --
+module m
+
+require rsc.io/quote v1.5.2
+
+replace rsc.io/quote v1.5.2 => example.com/quote v1.5.2
+
+-- use.go --
+package use
+
+import _ "rsc.io/quote"
+
+-- want --
+go: example.com/quote@v1.5.2: parsing go.mod:
+	module declares its path as: rsc.io/Quote
+	        but was required as: rsc.io/quote