Introduce attribute-based components (#478)

We introduce new attribute macros to mark methods or functions as
constructors:

- `#[pavex::constructor]`
- `#[pavex::request_scoped]`
- `#[pavex::transient]`
- `#[pavex::singleton]`

We also introduce a new `Blueprint` method, `import`, to determine which
annotated constructors are in scope at any given time, mimicking the
behaviour of Rust's `use` (as well as its syntax).

## Internal refactors

### Diagnostics

Beyond the changes required to find, validate and consider the new
components, I also had to extensively rework our diagnostic system to
provide a higher-level API, capable of abstracting over the way a
component was registered.
The outcome is a net positive change: working with spans is a lot easier
now, and the code has clearer intent than before.

### `OrderedCallGraph`

In order to simplify debugging, `OrderedCallGraph` has been reworked to
embed the ordering the in the node indices rather than using an external
`id <> position` mapping. This caused a lot of churn in our diagnostic
snapshots.

Author: LukeMathWalker

CHANGELOG.md

@@ -9,15 +9,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [0.1.80](https://github.com/LukeMathWalker/pavex/compare/0.1.79...0.1.80) - 2025-03-22
 
-
 ### 🐛 Bug Fixes
+
 - Always check if pavexc dependencies are installed (by @LukeMathWalker) - #475
 - Fix the path to the configuration folder in the Dockerfile of the starter project (by @LukeMathWalker) - #475
 
-
 ### Contributors
 
-* @LukeMathWalker
+- @LukeMathWalker
 
 ## [0.1.79](https://github.com/LukeMathWalker/pavex/compare/0.1.78...0.1.79) - 2025-03-14
 

doc_examples/quickstart/05-error.snap

@@ -18,6 +18,6 @@
   │  12 │     if let UserAgent::Unknown = user_agent {
   │     ╰────
   │   help: Register a constructor for `app::user_agent::UserAgent`.
-  [31m│[0m [36m  help: [0mAlternatively, use `Blueprint::prebuilt` to add a new input
+  [31m│[0m   [36mhelp:[0m Alternatively, use `Blueprint::prebuilt` to add a new input
   │         parameter of type `app::user_agent::UserAgent` to the (generated)
   │         `ApplicationState::new` method.

doc_examples/quickstart/07-error.snap

@@ -10,4 +10,4 @@
   │     ·                The fallible constructor was registered here
   │  14 │ 
   │     ╰────
-  [31m│[0m [36m  help: [0mAdd an error handler via `.error_handler`
+  [31m│[0m   [36mhelp:[0m Add an error handler via `.error_handler`