Adds a check so that module validation fails if there are any
assignments to immutable identifiers (e.g., module name, module
parameters, stdlib imports.)

BUG= https://bugs.chromium.org/p/chromium/issues/detail?id=640194

Review-Url: https://codereview.chromium.org/2268363002
Cr-Commit-Position: refs/heads/master@{#38838}