cmd/go: clarify error text for module path mismatch

This error occurs when a module is loaded with one name (for example,
github.com/golang/lint) but declares a different path in its go.mod
(golang.org/x/lint). The current text "unexpected module path" is
confusing. It doesn't explain why the path was unexpected, and it's
not clear what was expected.

With this change, the error text includes the module and version
containing the go.mod file with the error, the declared module path,
and the loaded module path. The paths are vertically aligned so
differences are visually obvious. As with other module version errors,
the shortest chain of requirements is printed.

This change supercedes CL 158477.

Fixes #28489

Change-Id: Ieb07d00bcae182376d7be6aad111c84fbf784354
Reviewed-on: https://go-review.googlesource.com/c/go/+/185985
Run-TryBot: Jay Conrod <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>

Author: Jay Conrod

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

@@ -1136,7 +1136,9 @@ func (r *mvsReqs) required(mod module.Version) ([]module.Version, error) {
 		return nil, module.VersionError(mod, errors.New("parsing go.mod: missing module line"))
 	}
 	if mpath := f.Module.Mod.Path; mpath != origPath && mpath != mod.Path {
-		return nil, module.VersionError(mod, fmt.Errorf("parsing go.mod: unexpected module path %q", mpath))
+		return nil, module.VersionError(mod, fmt.Errorf(`parsing go.mod:
+	module declares its path as: %s
+	          but was loaded as: %s`, mod.Path, mpath))
 	}
 	if f.Go != nil {
 		r.versions.LoadOrStore(mod, f.Go.Version)

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

@@ -57,19 +57,29 @@ import (
 func Test(t *testing.T) {}
 -- update-main-expected --
 go get: example.com/badchain/[email protected] updating to
-	example.com/badchain/[email protected]: parsing go.mod: unexpected module path "example.com/badchain/wrong"
+	example.com/badchain/[email protected]: parsing go.mod:
+	module declares its path as: example.com/badchain/c
+	          but was loaded as: example.com/badchain/wrong
 -- update-a-expected --
 go get: example.com/badchain/[email protected] requires
 	example.com/badchain/[email protected] requires
-	example.com/badchain/[email protected]: parsing go.mod: unexpected module path "example.com/badchain/wrong"
+	example.com/badchain/[email protected]: parsing go.mod:
+	module declares its path as: example.com/badchain/c
+	          but was loaded as: example.com/badchain/wrong
 -- list-expected --
 go: example.com/badchain/[email protected] requires
 	example.com/badchain/[email protected] requires
-	example.com/badchain/[email protected]: parsing go.mod: unexpected module path "example.com/badchain/wrong"
+	example.com/badchain/[email protected]: parsing go.mod:
+	module declares its path as: example.com/badchain/c
+	          but was loaded as: example.com/badchain/wrong
 -- list-missing-expected --
 go: m/use imports
-	example.com/badchain/c: example.com/badchain/[email protected]: parsing go.mod: unexpected module path "example.com/badchain/wrong"
+	example.com/badchain/c: example.com/badchain/[email protected]: parsing go.mod:
+	module declares its path as: example.com/badchain/c
+	          but was loaded as: example.com/badchain/wrong
 -- list-missing-test-expected --
 go: m/testuse tested by
 	m/testuse.test imports
-	example.com/badchain/c: example.com/badchain/[email protected]: parsing go.mod: unexpected module path "example.com/badchain/wrong"
+	example.com/badchain/c: example.com/badchain/[email protected]: parsing go.mod:
+	module declares its path as: example.com/badchain/c
+	          but was loaded as: example.com/badchain/wrong