Make private modules clearly private

[dfreeman]

Overview

This change looks large but is mainly moving blocks of text between files. The primary goal is to make it explicit in @glint/template and the environment packages when a module is meant to be considered an implementation detail.

Details

In the case of @glint/template I also took this as an opportunity to rationalize the layout and prune some long-outdated comments, as the contents of that package have evolved in a very piecemeal way over time.

While every module in @glint/template is now nested under -private, there are three main entrypoints for other Glint packages:

• @glint/template/-private/dsl contains the translated template DSL used by the output of @glint/transform that environment packages re-export with their own additions/tweaks
• @glint/template/-private/keywords contains the definition of shared keywords like {{#let}} that are relevant to any environment
• @glint/template/-private/integration contains the types necessary to declare how a class or other entity integrates with Glint's template layer (previously these types were scattered throughout, making such declarations a rat's nest of random imports)

For the environment packages, I shifted their DSL types and the environment definitions themselves under -private. For ember-loose, this also meant moving the registry module out into the package root, which is a nicer import anyway I think.

[jamescdavis]

💯