go.tools/go/loader: slashify the name of packages found by walking the file tree.

Otherwise on Windows the enumerated package "net\\http" will
be distinct from the imported package "net/http" leading to
strange errors.  (A similar bug was fixed in go/ssa/stdlib_test.go.)

Fixes golang/go#7189

LGTM=gri
R=gri
CC=golang-codereviews
https://golang.org/cl/86170043

go/loader/stdlib_test.go

@@ -35,7 +35,7 @@ func allPackages() []string {
 			return filepath.SkipDir
 		}
 		if info.IsDir() {
-			pkg := strings.TrimPrefix(path, root)
+			pkg := filepath.ToSlash(strings.TrimPrefix(path, root))
 			switch pkg {
 			case "builtin", "pkg":
 				return filepath.SkipDir // skip these subtrees