Improve global to import transform

[lemonmade]

Fixes #99. Maybe fixes #95. This PR fixes files importing themselves by checking the expose directive (so, it must come after the default export transform, which it does), and adds a global cache that I believe will speed up the lookups between transforms.

cc/ @bouk @GoodForOneFare @Fandy

[bouk]

nice 👍

[GoodForOneFare]

I think the snake case name will make it through to imports here.

[GoodForOneFare]

My bad, the snake case is getting through via imports.set at line 104.

          const file = getDeclaringFile(member);
          if (file === null) { return null; }
          const name = findLastMember(node).name;
          imports.set(member, {file, name});  <------------ here.
          return name;

[GoodForOneFare]

Okay, I was reading the diff the wrong way round 😥 This is really fixing some imports where the snake case version was sneaking in. That's good!

Before/after this transform:

-import ApplePayCertificatePoller from 'admin/modules/apple_pay_certificate_poller';
+import ApplePayCertificatePoller from 'admin/modules/apple-pay-certificate-poller';

[GoodForOneFare]

Code looks great. Just the snake case thing to fix before 🚢

I did a quick esify runthrough and everything was done in less time than the global import transform used to take on its own!! That's an outstanding result for a few shuffled lines 😁 Thank you!