[python][backends] Setup sys.modules with any loaded backend (triton-lang#6468)

From [importlib's
docs](https://docs.python.org/3.13/library/importlib.html#importing-a-source-file-directly)
you're supposed to setup `sys.modules` with the imported module after
you import it, and the name should be the fully qualified import path.

Besides following the docs, this enables any external user to
access/reference a backend module (otherwise there's no easy way--
meaning without traversing the `gc`-- to access those modules).

Author: danzimm

python/triton/backends/__init__.py

@@ -1,6 +1,7 @@
-import os
 import importlib.util
 import inspect
+import os
+import sys
 from dataclasses import dataclass
 from .driver import DriverBase
 from .compiler import BaseBackend
@@ -9,6 +10,7 @@
 def _load_module(name, path):
     spec = importlib.util.spec_from_file_location(name, path)
     module = importlib.util.module_from_spec(spec)
+    sys.modules[name] = module
     spec.loader.exec_module(module)
     return module
 
@@ -40,8 +42,8 @@ def _discover_backends():
             continue
         if name.startswith('__'):
             continue
-        compiler = _load_module(name, os.path.join(root, name, 'compiler.py'))
-        driver = _load_module(name, os.path.join(root, name, 'driver.py'))
+        compiler = _load_module(f"triton.backends.{name}.compiler", os.path.join(root, name, 'compiler.py'))
+        driver = _load_module(f"triton.backends.{name}.driver", os.path.join(root, name, 'driver.py'))
         backends[name] = Backend(_find_concrete_subclasses(compiler, BaseBackend),
                                  _find_concrete_subclasses(driver, DriverBase))
     return backends