python: Insert gem5's m5 importer at the front of sys.meta_path.

This is equivalent to putting a path first in $PATH, ie it will make
sure that m5.foo resolves to gem5's internal m5 package, and not
whatever directory named m5 happens to be nearby.

Change-Id: I762d47da8ed8e4cfdf03e412d7dd7cc91a691b58
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49415
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/python/importer.py

@@ -77,4 +77,4 @@ class CodeImporter(object):
 import sys
 importer = CodeImporter()
 add_module = importer.add_module
-sys.meta_path.append(importer)
+sys.meta_path.insert(0, importer)