Auto merge of #133461 - ferrocene:add-copyright-files-to-dist, r=Kobzol

Add COPYRIGHT-*.html files to distribution and update `COPYRIGHT`

* Updates the `COPYRIGHT` file to describe how we actually do things now, and removes the licence text from it as they are stored elsewhere.
* dist tarballs get all of the files in `LICENSES/*`.
  * This folder is managed by `reuse` and each file exists because we refer to the licence somewhere in our tree. We should be supplying these licence texts to anyone who obtains a copy of the source code and now we do.
* The binary rust tarball gets `COPYRIGHT.html` and `COPYRIGHT-library.html`, which are auto-generated files that describe the licence information for both the in-tree source files used to build the Rust toolchain, and the out-of-tree dependencies we used to build the toolchain.
   * The other binary tarballs are unchanged, for now. In future you need to make a call whether to ship multiple version of COPYRIGHT.html, or whether to try and make, for example, a cargo-specific COPYRIGHT.html file.
* The `LICENSE-MIT` file now includes a blanket copyright statement, as the text indicates that it should and because users will expect to know who owns the copyright of the material they have been given (even if the answer is 'lots of people').

try-job: x86_64-fuchsia

Author: bors