Rewrite the COPYRIGHT file

[brson]

This file may be more misleading than not at this point. It has not been kept up to date. I'm not sure the best thing to do here, but maintenance needs to be a concern, particularly now that the flow of crates into tree is opened. Listing the license of every single thing in tree is too hard for humans to do manually.

[xen0n]

Something like the NOTICE aggregation system used by AOSP should be OK. But keeping things synced in-tree may become a problem as the system generates the result NOTICE at build-time.

[steveklabnik]

docs team triage: tagging p-medium

[frewsxcv]

Some changes to COPYRIGHT were done in #43498

[hsivonen]

It would be useful for people who ship compiler output but who don't ship the compiler with the output if the COPYRIGHT file clearly distinguished between licenses that apply to the compiler and licenses that apply to compiler output.

For the latter case, it would also be useful to characterize under what condition certain licenses apply to the compiler output. One would hope the if jemalloc is disabled, the jemalloc license doesn't apply and if compiling with panic=abort, the libbacktrace license doesn't apply. #48370 suggests this might not be as simple as one could imagine, though, which makes documenting it even more valuable to people who ship compiler output.