building: have MERGE handle potential path prefix for extensions

As we add lib-dynload/ path prefix for python extensions coming
from that directory, we need to adjust relative path computation
in MERGE to handle potential path prefix in extension name.

This could be avoided if path prefix was added as package name
(i.e., with dot instead of path separator), but having it as
path component makes it more explicit and easier to spot.

PyInstaller/building/api.py

@@ -830,7 +830,11 @@ def _set_dependencies(self, analysis, path):
                     # relative path needs to be reconstructed from the
                     # name components.
                     if tpl[2] == 'EXTENSION':
-                        ext_components = tpl[0].split('.')[:-1]
+                        # Split on os.path.sep first, to handle additional
+                        # path prefix (e.g., lib-dynload)
+                        ext_components = tpl[0].split(os.path.sep)
+                        ext_components = ext_components[:-1] \
+                            + ext_components[-1].split('.')[:-1]
                         if ext_components:
                             rel_path = os.path.join(*ext_components)
                         else: