Attempt to fix group construction in Windows

Author: JukkaL

mypyc/build.py

@@ -465,7 +465,10 @@ def construct_groups(
         groups = []
         used_sources = set()
         for files, name in separate:
-            group_sources = [src for src in sources if src.path in files]
+            normalized_files = {os.path.normpath(f) for f in files}
+            group_sources = [
+                src for src in sources if os.path.normpath(src.path) in normalized_files
+            ]
             groups.append((group_sources, name))
             used_sources.update(group_sources)
         unused_sources = [src for src in sources if src not in used_sources]