* Only bind module.exports if no local definition exists
Note that this uses `lookupSymbolForNameWorker`, which is really a
best-effort check since it only knows about symbols that it has already
encountered.
As a side-effect, even when `module` is bound as part of a
`module.exports` reference, it only declares it once instead of one
declaration per reference.
* Only type module.exports inside module files
It is an error inside script files, but the binder sometimes creates a
ModuleExports symbol because we doesn't know whether we have a commonjs
module until after binding is done.
* Only bind module.exports in a commonjs module
Note that this, too, is a best-effort check since evidence of
commonjs-ness may be found after a *reference* to module.exports. (A
reference to module.exports alone is not enough evidence that a file is
commonjs. It has to have an assignment to it.)
