Earlier work to make "terraform init" interruptible made the getproviders
package context-aware in order to allow provider installation to be cancelled.

Here we make a similar change for module installation, which is now also
cancellable with SIGINT. This involves plumbing context through initwd and
getmodules. Functions which can make network requests now include a context
parameter whose cancellation cancels those requests.

Since the module installation code is shared, "terraform get" is now
also interruptible during module installation.