Only import types from declared dependencies

[nicolo-ribaudo]

Q	A
Fixed Issues?	Fixes #1, Fixes #2
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

This PR makes sure that all our import types only reference packages that are listed in dependencies/peerDependencies. This is done mainly in two ways:

1. plugins all peerDepend on @babel/core, so they can import NodePath/Visitor/Scope/types from there rather than from @babel/traverse/@babel/types
2. helpers can depend on @babel/traverse/@babel/types: if a helper expects a NodePath as a parameter, you already have to have @babel/traverse/@babel/types in your dependencies so this is not causing extra deps.

To (2) there are some exceptions:

• babel-helper-builder-react-jsx and babel-helper-plugin-utils only make sense when using with a full plugin (and not directly with parser+traverse), so I added @babel/core as a peer dependency (but only for Babel 8)
• babel-helper-fixtures is also only needed when testing plugins, so I added @babel/core as a peer dep

This PR doesn't lint @babel/standalone and @babel/parser since those are bundled, but we should probably add @babel/types as a dependency of @babel/parser. I don't really like doing it since it's perfectly reasonable to use @babel/parser without also using @babel/types, but on the other hand if the dependency is only used in import type it won't actually affect anything at runtime (it won't be loaded) so adding it is not too bad.

There is one remaining lint error fixed by #16493.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/57017

[nicolo-ribaudo]

There are 4 packages whose types circularly depend on @babel/traverse. Lets just ignore those errors for now (@babel/traverse will be in the dependencies anyway), but I will think of a better solution before going stable.