Remove `source` field in favor of `sources`

# Summary

Remove the `source` field and instead have folks just populate one item in the `sources` array.

# Motivation

There's a lot of logic just built around toggling between `source` and `sources` in an app's `spec` field. It complicates tests and reduces readability.

# Proposal

Drop the field, fix all current logic which tests for `sources` length.