fix(package): resolve source paths relative to package root

setup.py

@@ -49,16 +49,19 @@ REQUIRED_VENDOR_FILES = (
 
 
 def _source_directory():
+    """Find package root and return it relative to the current working directory."""
     candidates = []
     if globals().get('__file__'):
         candidates.append(os.path.dirname(os.path.abspath(__file__)))
     for name in ('basedir', 'package_dir'):
         if globals().get(name):
             candidates.append(os.path.abspath(globals()[name]))
-    candidates.append(os.getcwd())
+    current_directory = os.getcwd()
+    candidates.append(current_directory)
     for candidate in candidates:
-        if os.path.isfile(os.path.join(candidate, 'WAPT', 'control')):
-            return candidate
+        relative_candidate = os.path.relpath(os.path.abspath(candidate), current_directory)
+        if os.path.isfile(os.path.join(relative_candidate, 'WAPT', 'control')):
+            return relative_candidate
     raise RuntimeError('Cannot locate WAPT source directory (WAPT/control not found).')