Add import grouping to the styleguide

[ekcasey]

We should describe how import statements should be grouped in the styleguide. This seems like a great example of the kind of detail that can cause unnecessary back-and-forth on a PR. I would suggest we organize imports in one delaration with three groups:

1. stdlib
2. external (imported from another repo or module)
3. internal (same module)

This can be configured in Goland with the following settings:

Or automated with the following command:

goimports -l -w -local <module> .

e.g

goimports -l -w -local github.com/paketo-buildpacks/environment-variables .