red-knot: move symbol lookups in symbol.rs (#16152)

## Summary

This PR does the following:
* Moves the following from `types.rs` in `symbol.rs`:
	* `symbol`
	* `global_symbol`
	* `imported_symbol`
	* `symbol_from_bindings`
	* `symbol_from_declarations`
	* `SymbolAndQualifiers`
	* `SymbolFromDeclarationsResult`
* Moves the following from `stdlib.rs` in `symbol.rs` and removes
`stdlib.rs`:
	* `known_module_symbol`
	* `builtins_symbol`
	* `typing_symbol` (only for tests)
	* `typing_extensions_symbol`
	* `builtins_module_scope`
	* `core_module_scope`
* Add `symbol_from_bindings_impl` and `symbol_from_declarations_impl` to
keep `RequiresExplicitReExport` an implementation detail
* Make `declaration_type` a `pub(crate)` as it's required in
`symbol_from_declarations` (`binding_type` is already `pub(crate)`

The main motivation is to keep the implementation details private and
only expose an ergonomic API which uses sane defaults for various
scenario to avoid any mistakes from the caller. Refer to
https://github.com/astral-sh/ruff/pull/16133#discussion_r1955262772,
https://github.com/astral-sh/ruff/pull/16133#issue-2850146612 for
details.

crates/red_knot_python_semantic/src/lib.rs

@@ -22,7 +22,6 @@ mod python_platform;
 pub mod semantic_index;
 mod semantic_model;
 pub(crate) mod site_packages;
-mod stdlib;
 mod suppression;
 pub(crate) mod symbol;
 pub mod types;