resources: Create a common ResourceFinder interface

And make both .Resources and resources implement it.

This gets us 2 new methods/functions, so you can now also do:

* .Resources.Get
* resources.ByType

Note that GetRemote is not covered by this interface, as that is only available as a global template function.

Fixes #8653

common/types/hstring/stringtypes_test.go

@@ -21,7 +21,7 @@ import (
 	"github.com/spf13/cast"
 )
 
-func TestStringTypes(t *testing.T) {
+func TestRenderedString(t *testing.T) {
 	c := qt.New(t)
 
 	// Validate that it will behave like a string in Hugo settings.