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.
This commit blanket renames the `rustc_macro` infrastructure to `proc_macro`,
which reflects the general consensus of #35900. A follow up PR to Cargo will be
required to purge the `rustc-macro` name as well.
