We were broken by rust-lang/cargo#10343.

See rust-lang/cargo#11425

Fixes #4519

Improve strategy for selecting targets to be scraped for examples

### What does this PR try to resolve?

After #10343, we have identified a clear set of conditions for whether a particular target should be considered by `-Zrustdoc-scrape-examples`. These conditions are described more clearly in #11425.

However, after some testing with complex Cargo workspaces (e.g. [wasmtime](https://github.com/bytecodealliance/wasmtime/)), I realized that the current approach of modifying the `CompileFilter` did not correctly implement this new specification. For example, a target with `doc = false` should not be scraped by default since it is not a documented unit, but the current approach would potentially include such a target for scraping.

This PR provides a new approach which I believe correctly implements the specification:
1. `generate_targets` is called with the same parameters except the `mode` which becomes `CompileMode::Docscrape` instead of `CompileMode::Doc`. `filter_default_targets` generates the same targets for `Docscrape` as for `Doc`.
2. Inside `generate_targets`, an initial set of `Proposal`s are created. This set of proposals is extended with further proposals based on targets identified as `doc-scrape-examples = true`, or Example targets where possible.

This PR subsumes #11423, and also fixes #10571.

### How should we test and review this PR?

I have added another test `docscrape::only_scrape_documented_targets` to verify that only documented or explicitly-enabled targets are included for scraping.

r? `@weihanglo`

9 commits in e027c4b5d25af2119b1956fac42863b9b3242744..f6e737b1e3386adb89333bf06a01f68a91ac5306
2022-11-25 19:44:46 +0000 to 2022-12-02 20:21:24 +0000
- Refactor generate_targets into separate module (rust-lang/cargo#11445)
- Improve file found in multiple build targets warning (rust-lang/cargo#11299)
- Error when precise without -p flag (rust-lang/cargo#11349)
- Improve strategy for selecting targets to be scraped for examples (rust-lang/cargo#11430)
- Aware of compression ratio for unpack size limit (rust-lang/cargo#11337)
- Add test for rustdoc-map generation when using sparse registries (rust-lang/cargo#11403)
- Add error message when `cargo fix` on an empty repo (rust-lang/cargo#11400)
- Store the sparse+ prefix in the SourceId for sparse registries (rust-lang/cargo#11387)
- Update documentation for -Zrustdoc-scrape-examples in the Cargo Book (rust-lang/cargo#11425)

Update cargo

9 commits in e027c4b5d25af2119b1956fac42863b9b3242744..f6e737b1e3386adb89333bf06a01f68a91ac5306
2022-11-25 19:44:46 +0000 to 2022-12-02 20:21:24 +0000
- Refactor generate_targets into separate module (rust-lang/cargo#11445)
- Improve file found in multiple build targets warning (rust-lang/cargo#11299)
- Error when precise without -p flag (rust-lang/cargo#11349)
- Improve strategy for selecting targets to be scraped for examples (rust-lang/cargo#11430)
- Aware of compression ratio for unpack size limit (rust-lang/cargo#11337)
- Add test for rustdoc-map generation when using sparse registries (rust-lang/cargo#11403)
- Add error message when `cargo fix` on an empty repo (rust-lang/cargo#11400)
- Store the sparse+ prefix in the SourceId for sparse registries (rust-lang/cargo#11387)
- Update documentation for -Zrustdoc-scrape-examples in the Cargo Book (rust-lang/cargo#11425)