Create Way to force package even if filepath exists

Fixes: #268

Author: graingert

coverage/config.py

@@ -195,6 +195,7 @@ def __init__(self):
         self.run_include = None
         self.run_omit = None
         self.source = None
+        self.source_pkgs = []
         self.timid = False
         self._crash = None
 
@@ -361,6 +362,7 @@ def copy(self):
         ('run_include', 'run:include', 'list'),
         ('run_omit', 'run:omit', 'list'),
         ('source', 'run:source', 'list'),
+        ('source_pkgs', 'run:source_pkgs', 'list'),
         ('timid', 'run:timid', 'boolean'),
         ('_crash', 'run:_crash'),
 

coverage/inorout.py

@@ -132,6 +132,7 @@ def __init__(self, warn, debug):
 
     def configure(self, config):
         """Apply the configuration to get ready for decision-time."""
+        self.source_pkgs.extend(config.source_pkgs)
         for src in config.source or []:
             if os.path.isdir(src):
                 self.source.append(canonical_filename(src))