Inline package metadata in main package.json file

Atom currently ships with 77 bundled packages.

So each time a window is opened, 77 `package.json` files are read/parsed, ~25 package keymap JSON files are read/parsed, and ~25 package menu JSON files are read/parsed.

In order to minimize the read/parse time during startup, inline all this content into the main `package.json` file under a special `_atomPackages` key.

For me this drops 25-50ms off of package load/activate time and should be more for slower machines and non-SSD drives.

Also this removes ~175 files/folders from the distribution which makes it a little leaner.