Get rid of imp, started giving deprecation warning every time in Python 3.7 in Ubuntu 19.04.

Please python stop torturing me with refactors.

Make ./run -u blow up if executable not found, otherwise I go crazy.

Get ./test-gdb back to life after the ./run relative path refactor, forgot to test this.

build-userland-in-tree

@@ -1,18 +1,11 @@
 #!/usr/bin/env python3
 
-import imp
 import os
 import subprocess
 
-git_root = subprocess.check_output([
-    'git',
-    'rev-parse',
-    '--show-toplevel',
-]).decode().rstrip()
-build_userland = imp.load_source(
-    'build_userland',
-    os.path.join(git_root, 'build-userland')
-)
+import common
+
+build_userland = common.import_path_relative_root('build-userland')
 
 class Main(build_userland.Main):
     def __init__(self):