go/internal/load: fix bad error message for local imports in module mode

src/cmd/go/internal/load/pkg.go

@@ -752,7 +752,9 @@ func loadImport(ctx context.Context, opts PackageOpts, pre *preload, path, srcDi
 	if p.Internal.Local && parent != nil && !parent.Internal.Local {
 		perr := *p
 		var err error
-		if path == "." {
+		if cfg.ModulesEnabled {
+			err = ImportErrorf(path, "local import %q not supported in module mode", path)
+		} else if path == "." {
 			err = ImportErrorf(path, "%s: cannot import current directory", path)
 		} else {
 			err = ImportErrorf(path, "local import %q in non-local package", path)

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

@@ -0,0 +1,17 @@
+! go build
+stderr 'main.go:4:2: local import "./foo" not supported in module mode$'
+
+-- foo/foo.go --
+package foo
+-- go.mod --
+module example.com/m
+
+go 1.17
+-- main.go --
+package main
+
+import (
+	_ "./foo"
+)
+
+func main() {}

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

@@ -0,0 +1,27 @@
+env GO111MODULE=off
+
+# Control case: in GOPATH mode,
+# a/a.go can import package "./b" using a relative path.
+go build a/a.go
+! stderr .
+
+# package "a" itself cannot import "./b": "a" has a non-local
+# import path, so its imports must also have non-local import paths.
+! go build a
+stderr '^a[/\\]a.go:3:8: local import "./b" in non-local package$'
+
+# package "c" imports itself, which is not allowed.
+! go build c
+stderr '^c[/\\]c.go:3:8: .: cannot import current directory$'
+
+
+-- a/a.go --
+package a
+
+import _ "./b"
+-- a/b/b.go --
+package b
+-- c/c.go --
+package c
+
+import _ "."