Unify coreAliasing, externalRuntime and externalRegenerator

Merge all three transformers into the `selfContained` transformer,
as discussed in Gitter.  Name may not be final.

test/fixtures/transformation/optional-self-contained/runtime-commonjs/actual.js

@@ -0,0 +1 @@
+import * as foo from "someModule";

test/fixtures/transformation/optional-self-contained/runtime-commonjs/expected.js

@@ -0,0 +1,9 @@
+"use strict";
+
+var _to5Runtime = require("6to5-runtime/6to5");
+
+var _core = require("6to5-runtime/core-js");
+
+var _regeneratorRuntime = require("6to5-runtime/regenerator");
+
+var foo = _to5Runtime.interopRequireWildcard(require("someModule"));