Fix: Blender as Python Module shared library directory wrong

This could affect for example the USD and MaterialX Python modules
that are now bundled, and need appropriate paths to their libraries.

Ref #134676

Pull Request: https://projects.blender.org/blender/blender/pulls/134937

scripts/site/sitecustomize.py

@@ -7,15 +7,25 @@
 import sys
 import os
 
-exe_dir, exe_file = os.path.split(sys.executable)
-is_python = exe_file.startswith("python")
-
 # Path to Blender shared libraries.
 shared_lib_dirname = "blender.shared" if sys.platform == "win32" else "lib"
-if is_python:
-    shared_lib_dir = os.path.abspath(os.path.join(exe_dir, "..", "..", "..", shared_lib_dirname))
+
+if os.path.basename(__file__) == "bpy_site_customize.py":
+    # Blender as Python Module.
+    is_python = True
+    shared_lib_dir = os.path.abspath(os.path.join(os.path.dirname(__file__), "..", "..", ".."))
+    # On Windows no subdirectory is used.
+    if sys.platform != "win32":
+        shared_lib_dir = os.path.join(shared_lib_dir, shared_lib_dirname)
 else:
-    shared_lib_dir = os.path.abspath(os.path.join(exe_dir, shared_lib_dirname))
+    exe_dir, exe_file = os.path.split(sys.executable)
+    is_python = exe_file.startswith("python")
+    if is_python:
+        # Python executable bundled with Blender.
+        shared_lib_dir = os.path.abspath(os.path.join(exe_dir, "..", "..", "..", shared_lib_dirname))
+    else:
+        # Blender executable.
+        shared_lib_dir = os.path.abspath(os.path.join(exe_dir, shared_lib_dirname))
 
 if sys.platform == "win32":
     # Directory for extensions to find DLLs.