dist: Re-work how we describe the licence of Rust in our distributions

) add COPYRIGHT*.html files to the rustc binary distribution
) add contents of LICENSE folder to dist tarballs, because some of our in-tree licences will require that the license text is reproduced.
) The wording of COPYRIGHT is adjusted to not include license text (`reuse` ensures that it's in the LICENSE folder)
) A blanket copyright notice is added to LICENCE-MIT as required by the text.

The general approach is that the license statements are now compiled using a tool in CI (generate-copyright), and you get either:

* the source code (COPYRIGHT, LICENCE-APACHE, LICENCE-MIT, REUSE.toml and the LICENCES folder), or
* the compiled version (COPYRIGHT.html, COPYRIGHT-library.html and the LICENCES folder).

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