swift234

Disallow importing other Swift source files as modules.

Because this is useful in testing, I've left in a frontend option
-enable-source-import for both swift and swift-ide-test that sidesteps the
module restriction. Right now, though, this is the right thing to avoid
users running into strange issues when they import another file within
their module and Swift treats it as a separate module.

<rdar://problem/15937521>

Swift SVN r13248