Don't fail loading subprojects if subprojects_dir is in a subdirectory

mesonbuild/interpreter.py

@@ -33,6 +33,7 @@ from .modules import ModuleReturnValue
 import os, sys, shutil, uuid
 import re, shlex
 from collections import namedtuple
+from pathlib import PurePath
 
 import importlib
 
@@ -2963,11 +2964,16 @@ different subdirectory.
     def evaluate_subproject_info(self, path_from_source_root, subproject_dirname):
         depth = 0
         subproj_name = ''
-        segs = path_from_source_root.split(os.path.sep)
-        while segs and segs[0] == subproject_dirname:
-            depth += 1
-            subproj_name = segs[1]
-            segs = segs[2:]
+        segs = PurePath(path_from_source_root).parts
+        segs_spd = PurePath(subproject_dirname).parts
+        while segs and segs[0] == segs_spd[0]:
+            if len(segs_spd) == 1:
+                subproj_name = segs[1]
+                segs = segs[2:]
+                depth += 1
+            else:
+                segs_spd = segs_spd[1:]
+                segs = segs[1:]
         return (depth, subproj_name)
 
     # Check that the indicated file is within the same subproject