Rollup merge of #87282 - pietroalbini:refactor-extended, r=Mark-Simulacrum

Ensure `./x.py dist` adheres to `build.tools`

According to `config.toml.example`, the way to produce dist artifacts for both the compiler and a *subset* of tools would be to enable the extended build and manually specify the list of tools to build:

```toml
[build]
extended = true
tools = ["cargo", "rustfmt"]
```

This works as expected for `./x.py build` and `./x.py install`, but *not* for `./x.py dist`. Before this PR `./x.py dist` simply ignored the contents of `build.tools`, building just rustc/rustdoc if `build.extended = false` and all of the tools otherwise. This PR does two things:

* Changes `./x.py dist extended` to only build the tools defined in `build.tools`, if `build.tools` is not empty. The rest of the extended step was refactored to simplify the code.
* Changes how dist jobs for tools are gated: instead of `assert!(builder.config.extended)` to prevent tools from being built with `build.extended = false`, tools are simply built by default depending on `build.extended` and `build.tools`. This also enables to **explicitly** dist tools even with `build.extended = false`.

This PR is best reviewed commit-by-commit.

Fixes #86436

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