Add a fix/warning so that stale .pyc files don't kill Spack.

- Can't think of a better way to do this.
- The externals integration will cause spack to die in weird ways for
  users who just pull from develop.

bin/spack

@@ -41,6 +41,21 @@ sys.path.insert(0, SPACK_LIB_PATH)
 SPACK_EXTERNAL_LIBS = os.path.join(SPACK_LIB_PATH, "external")
 sys.path.insert(0, SPACK_EXTERNAL_LIBS)
 
+# Quick and dirty check to clean orphaned .pyc files left over from
+# previous revisions.  These files were present in earlier versions of
+# Spack, were removed, but shadow system modules that Spack still
+# imports.  If we leave them, Spack will fail in mysterious ways.
+# TODO: more elegant solution for orphaned pyc files.
+orphaned_pyc_files = [os.path.join(SPACK_EXTERNAL_LIBS, n)
+                      for n in ('functools.pyc', 'ordereddict.pyc')]
+for pyc_file in orphaned_pyc_files:
+    if not os.path.exists(pyc_file):
+        continue
+    try:
+        os.remove(pyc_file)
+    except OSError as e:
+        print "WARNING: Spack may fail mysteriously. Couldn't remove orphaned .pyc file: %s" % pyc
+
 # If there is no working directory, use the spack prefix.
 try:
     working_dir = os.getcwd()