Define a class called PackageAPI and put it in its own file.

Summary:
The methods of the ad-hoc `api` object needed access to very few variables
from the surrounding lexical environment, and those few variables
(`sources`, `exports`, `uses`, `implies`) are better regarded as member
variables of the `PackageAPI` class.

Test Plan: `meteor test-packages`

Reviewers: Slava, dgreensp, glasser

Reviewed By: glasser

Differential Revision: https://phabricator.meteor.io/D30

docs/client/data.js

@@ -4523,7 +4523,7 @@ DocsData = {
     "options": [],
     "params": [
       {
-        "description": "<p>A function that takes in the package control 'api' object, which keeps track of dependencies and exports.</p>",
+        "description": "<p>A function that takes in the package control <code>api</code> object, which keeps track of dependencies and exports.</p>",
         "name": "func",
         "type": {
           "names": [
@@ -4602,7 +4602,7 @@ DocsData = {
     "options": [],
     "params": [],
     "scope": "global",
-    "summary": "The API object passed into the Packages.onUse function."
+    "summary": "Type of the API object passed into the `Package.onUse` function."
   },
   "PackageAPI#addFiles": {
     "kind": "function",