Core: Ensure classes match their header filename

Also drop some unused files.

Renamed:
- `core/dvector.h` -> `pool_vector.h`
- `core/io/resource_import.h` -> `resource_importer.h`
- `core/sort.h` -> `sort_array.h`
- `core/string_db.h` -> `string_name.h`

Dropped:
- `core/allocators.h`
- `core/os/shell.h`
- `core/variant_construct_string.cpp`

core/variant.h

@@ -38,7 +38,6 @@
 #include "core/array.h"
 #include "core/color.h"
 #include "core/dictionary.h"
-#include "core/dvector.h"
 #include "core/io/ip_address.h"
 #include "core/math/aabb.h"
 #include "core/math/basis.h"
@@ -49,6 +48,7 @@
 #include "core/math/transform_2d.h"
 #include "core/math/vector3.h"
 #include "core/node_path.h"
+#include "core/pool_vector.h"
 #include "core/ref_ptr.h"
 #include "core/rid.h"
 #include "core/ustring.h"