Don't thread the package file data

[WesCossick]

Right now, we retrieve the package.json file's contents in the index.ts file and then thread that object through to steps that need it. Here's where we retrieve it, and here's where we thread it through.

We used to do this with the config file, too, but refactored our code to create and use a getConfig() function instead. This function only parses the config file once, and just returns the already-parsed version if it has it. That way, we can call the getConfig() function anywhere we need config values without a performance hit.

We should do essentially the same thing for the package.json file data, via a function called getPackageFile().

We'll probably also want to create a get-package-file.test.ts file with appropriate tests.