Auto merge of rust-lang#102565 - jyn514:refactor-build-manifest, r=Mark-Simulacrum

Refactor build-manifest to minimize the number of changes needed to add a new component

- Add all components to `PkgType`
- Automate functionality wherever possible, so functions often don't have to be manually edited
- Where that's not possible, use exhaustive matches on `PkgType` instead of adding individual strings.
- Add documentation for how to add a component. Improve the existing documentation for how to test changes.

I tested locally that this generates an identical manifest before and after my change, as follows:
```sh
git checkout d44e142
cargo +nightly run --manifest-path src/tools/build-manifest/Cargo.toml build/dist build/manifest-before 1970-01-01 http://example.com nightly
git checkout refactor-build-manifest
cargo +nightly run --manifest-path src/tools/build-manifest/Cargo.toml build/dist build/manifest-before 1970-01-01 http://example.com nightly
sort -u build/manifest-before/channel-rust-nightly.toml | diff - <(sort -u build/manifest-after/channel-rust-nightly.toml)
```
I then verified by hand that the differences before sorting are inconsequential (mostly targets being slightly reordered).

The only change in behavior is that `llvm-tools` is now properly renamed to `llvm-tools-preview`:
```
; sort -u build/manifest-before/channel-rust-nightly.toml | diff - <(sort -u build/manifest-after/channel-rust-nightly.toml)
784a785
> [renames.llvm-tools]
894a896
> to = "llvm-tools-preview"
```

This is based on rust-lang#102241 and should not be merged before.

Author: bors