Separate the plugin code from non-plugin code to break a potential cycle in crates.
This will allow us to merge the new libproc_macro_tokens into libproc_macro.
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.
This updates the commit to use workspaces to use `cargo metadata` instead of
hardcoded lists about what to test. This should help us be resilient to updates
in the future on behalf of the crate DAG and minimize the amount of files that
need to be touched.
This involves hacking the code used to run cargo test on various
packages, because it reads Cargo.lock to determine which packages should
be tested. This change implements a blacklist, since that will catch new
crates when they are added in the future.
