Optimize the multilanguage build process

Work In Progress!

This commit makes a rework of the build and rebuild process to better suit a multi-site setup.

This also includes a complete overhaul of the site tests. Previous these were a messy mix that
were testing just small parts of the build chain, some of it testing code-paths not even used in
"real life". Now all tests that depends on a built site follows the same and real production code path.

See #2309
Closes #2211
Closes #477
Closes #1744

source/filesystem_test.go

@@ -22,7 +22,7 @@ import (
 )
 
 func TestEmptySourceFilesystem(t *testing.T) {
-	src := new(Filesystem)
+	src := &Filesystem{Base: "Empty"}
 	if len(src.Files()) != 0 {
 		t.Errorf("new filesystem should contain 0 files.")
 	}