Streamline module structure and imports

[PoignardAzur]

Module structure

Virtually every module should be private. The only public modules should inline modules that gather public-facing re-exports.

Most items should be exported from the root module, with no other public-facing export. This makes documentation more readable; readers don't need to click on multiple modules to find the item they're looking for.

There should be only three public modules:

• widgets
• commands
• test_widgets

Module files should not be foobar/mod.rs. Instead, they should _foobar.rs (thus in the parent folder); the full name is for readability, the leading underscore is so these names appear first in file hierarchies.

Imports

We should avoid use super::xxx imports, except for specific cases like unit tests.

Most imports in the code base should use the canonical top-level import.

Prelude

Masonry should have no prelude. Examples and documentation should deliberately have a list of imports that cover everything users will need, so users can copy-paste these lists.

[PoignardAzur]

Addressed by #848 and #843.