importing: make importlib_importer recognize .pyc cache (#13239)

Our importer was always parsing from source (which is considerably
slower) because the source size recorded in the .pyc file differed from
the size of the input file.

Override path_stats in the prepending importer to fool it into thinking
that the source size is the size *with* the prepended code.

lib/spack/spack/util/imp/importlib_importer.py

@@ -15,6 +15,12 @@ def __init__(self, full_name, path, prepend=None):
         super(PrependFileLoader, self).__init__(full_name, path)
         self.prepend = prepend
 
+    def path_stats(self, path):
+        stats = super(PrependFileLoader, self).path_stats(path)
+        if self.prepend:
+            stats["size"] += len(self.prepend) + 1
+        return stats
+
     def get_data(self, path):
         data = super(PrependFileLoader, self).get_data(path)
         if path != self.path or self.prepend is None: