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

mode

In module mode, if an import statement refers to a relative path it is
currently supposed to fail with relative import not supported
However, we seem to be missing test coverage for that error message,
and the actual error that is printed for that case is substantially different

Author: Yuuki77

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

@@ -754,6 +754,8 @@ func loadImport(ctx context.Context, opts PackageOpts, pre *preload, path, srcDi
 		var err error
 		if path == "." {
 			err = ImportErrorf(path, "%s: cannot import current directory", path)
+		} else if build.IsLocalImport(path) {
+			err = ImportErrorf(path, "%s: relative import not supported", 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: ./foo: relative import not supported'
+
+-- foo/foo.go --
+package foo
+-- go.mod --
+module example.com/m
+
+go 1.17
+-- main.go --
+package main
+
+import (
+	_ "./foo"
+)
+
+func main() {}