As the entry point for building the Rust compiler, a good user experience hinges
on this compiling quickly to get to the meat of the problem. To that end use
`#[cfg]`-specific dependencies to avoid building Windows crates on Unix and drop
the `regex` crate for now which was easily replacable with some string
searching.
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/`.
