This check turns out to be fairly expensive on Windows (~150ms), and having
it at import time means we're taking the hit every time we launch
memory_cache_http_server. We only need to do this if the Python files have
changed, so move it to runhooks.

The reason this check exists in the first place is that we've had problems in
the past where e.g. someone renames foo.py to foo/__init__.py. foo.pyc remains
on the filesystem of every Chromium checkout and causes a name conflict in
Python imports - foo can refer to the compiled module or the package.

BUG=388256
TEST=git rm a file in src/tools that has a .pyc; git commit; gclient runhooks; ensure .pyc doesn't exist; undo commit

Review URL: https://codereview.chromium.org/489693002

Cr-Commit-Position: refs/heads/master@{#291825}