This is a resurrection of #40776, combining their Windows setup with an
additional setup on Unix to set the program group's niceness to +10
(low-but-not-lowest priority) when the `low_priority` option is on.
A few changes are included here:
* The `winapi` and `url` dependencies were dropped. The source code for these
projects is pretty weighty, and we're about to vendor them, so let's not
commit to that intake just yet. If necessary we can vendor them later but for
now it shouldn't be necessary.
* The `--frozen` flag is now always passed to Cargo, obviating the need for
tidy's `cargo_lock` check.
* Tidy was updated to not check the vendor directory
Closes#34687
The organization in rustbuild was a little odd at the moment where the `lib.rs`
was quite small but the binary `main.rs` was much larger. Unfortunately as well
there was a `build/` directory with the implementation of the build system, but
this directory was ignored by GitHub on the file-search prompt which was a
little annoying.
This commit reorganizes rustbuild slightly where all the library files (the
build system) is located directly inside of `src/bootstrap` and all the binaries
now live in `src/bootstrap/bin` (they're small). Hopefully this should allow
GitHub to index and allow navigating all the files while maintaining a
relatively similar layout to the other libraries in `src/`.
