Small post-restructuring refactors

[sodic]

Still in progress, just wanted to save it

Release of Wasp 0.12.0 was pretty hectic and we've cut many corners. We should go back and clean up the some of technical debt where we think it makes sense.

All items are refactors (i.e., no visible changes to the user) and felt to small to warrant their own issue.

Here's a list of todos we've marked for refactoring:

• Reconsider GeneratedExternalCodeDir location
• Revisit the NpmDependencies file
• Revisit the generators TODOs:
  • SDK generator
  • Remove metnions of ext (we kept it to make conflict resolution easier) - For example, change extPublicDir terminology to just publicDir
• Fix small typo: Restructuring prototype RFC #1584 (comment)
• Revisit Implement the project install step #1602 (comment)