Fix issue #22 and #25: Calculate relative path for constraints reference

- processing.py: Calculate relative path from requirements-out directory to
  constraints-out file using os.path.relpath()
- Handles different directory scenarios (same dir, parent dir, subdirs)
- Falls back to absolute path if relpath fails (e.g., different drives on Windows)
- CHANGES.md: Document the fix

Author: jensens

CHANGES.md

@@ -11,6 +11,9 @@
 - Fix #46: Git tags in branch option are now correctly detected and handled during updates. Previously, updating from one tag to another failed because tags were incorrectly treated as branches.
   [jensens]
 
+- Fix #22 and #25: Constraints file path in requirements-out is now correctly calculated as a relative path from the requirements file's directory. This allows requirements and constraints files to be in different directories. Previously, the path was written from the config file's perspective, causing pip to fail when looking for the constraints file. On Windows, paths are now normalized to use forward slashes for pip compatibility.
+  [jensens]
+
 - Fix #53: Per-package target setting now correctly overrides default-target when constructing checkout paths.
   [jensens]
 

src/mxdev/processing.py

@@ -271,9 +271,26 @@ def write(state: State) -> None:
     logger.info(f"Write [r]: {cfg.out_requirements}")
     with open(cfg.out_requirements, "w") as fio:
         if constraints or cfg.overrides:
+            # Calculate relative path from requirements-out directory to constraints-out file
+            # This ensures pip can find the constraints file regardless of where requirements
+            # and constraints files are located
+            from pathlib import Path
+            import os
+
+            req_path = Path(cfg.out_requirements)
+            const_path = Path(cfg.out_constraints)
+
+            # Calculate relative path from requirements directory to constraints file
+            try:
+                constraints_ref = os.path.relpath(const_path, req_path.parent)
+            except ValueError:
+                # On Windows, relpath can fail if paths are on different drives
+                # In that case, use absolute path
+                constraints_ref = str(const_path.absolute())
+
             fio.write("#" * 79 + "\n")
             fio.write("# mxdev combined constraints\n")
-            fio.write(f"-c {cfg.out_constraints}\n\n")
+            fio.write(f"-c {constraints_ref}\n\n")
         write_dev_sources(fio, cfg.packages)
         fio.writelines(requirements)
         write_main_package(fio, cfg.settings)