discuss file system checking in FastBoot.require

Every time `FastBoot.require` is run, `execSync` is run no matter how many times you call it [here](https://github.com/ember-fastboot/fastboot/blob/b62e795c8c21c4a5dca09f2cf20e4367c843fc7b/src/ember-app.js#L101).
Can this be optimized away? Perhaps maintain a string list of the packages already looked up, and skip the file system if found.
I would like to avoid people having to worry about caching the result of this in user land.