Fix missing license files in published crates (except quiche)

[musicinmybrain]

Adds license files to published crates by using symbolic links to the top-level license file.

This is important because the chosen BSD-2-Clause license requires the license text to be included in source and binary distributions.

This was already handled for the quiche crate, but not for any of the others.

Before this PR:

$ git clone https://github.com/cloudflare/quiche.git
$ cd quiche
$ git submodule update --init --recursive
$ cd qlog
$ cargo publish --dry-run
$ tar -tvf ../target/package/qlog-0.15.2.crate
-rw-r--r-- 0/0              98 1970-01-01 01:00 qlog-0.15.2/.cargo_vcs_info.json
-rw-r--r-- 0/0            5426 1970-01-01 01:00 qlog-0.15.2/Cargo.lock
-rw-r--r-- 0/0            1259 1970-01-01 01:00 qlog-0.15.2/Cargo.toml
-rw-r--r-- 0/0             573 2006-07-24 02:21 qlog-0.15.2/Cargo.toml.orig
-rw-r--r-- 0/0            8895 2006-07-24 02:21 qlog-0.15.2/README.md
-rw-r--r-- 0/0            4528 2006-07-24 02:21 qlog-0.15.2/src/events/connectivity.rs
-rw-r--r-- 0/0            6698 2006-07-24 02:21 qlog-0.15.2/src/events/h3.rs
-rw-r--r-- 0/0           27405 2006-07-24 02:21 qlog-0.15.2/src/events/mod.rs
-rw-r--r-- 0/0            7538 2006-07-24 02:21 qlog-0.15.2/src/events/qpack.rs
-rw-r--r-- 0/0           21018 2006-07-24 02:21 qlog-0.15.2/src/events/quic.rs
-rw-r--r-- 0/0            2738 2006-07-24 02:21 qlog-0.15.2/src/events/security.rs
-rw-r--r-- 0/0           27447 2006-07-24 02:21 qlog-0.15.2/src/lib.rs
-rw-r--r-- 0/0            3700 2006-07-24 02:21 qlog-0.15.2/src/reader.rs
-rw-r--r-- 0/0           22216 2006-07-24 02:21 qlog-0.15.2/src/streamer.rs

After this PR:

$ cargo publish --dry-run
$ tar -tvf ../target/package/qlog-0.15.2.crate
-rw-r--r-- 0/0              98 1970-01-01 01:00 qlog-0.15.2/.cargo_vcs_info.json
-rw-r--r-- 0/0            1306 2006-07-24 02:21 qlog-0.15.2/COPYING
-rw-r--r-- 0/0            5426 1970-01-01 01:00 qlog-0.15.2/Cargo.lock
-rw-r--r-- 0/0            1259 1970-01-01 01:00 qlog-0.15.2/Cargo.toml
-rw-r--r-- 0/0             573 2006-07-24 02:21 qlog-0.15.2/Cargo.toml.orig
-rw-r--r-- 0/0            8895 2006-07-24 02:21 qlog-0.15.2/README.md
-rw-r--r-- 0/0            4528 2006-07-24 02:21 qlog-0.15.2/src/events/connectivity.rs
-rw-r--r-- 0/0            6698 2006-07-24 02:21 qlog-0.15.2/src/events/h3.rs
-rw-r--r-- 0/0           27405 2006-07-24 02:21 qlog-0.15.2/src/events/mod.rs
-rw-r--r-- 0/0            7538 2006-07-24 02:21 qlog-0.15.2/src/events/qpack.rs
-rw-r--r-- 0/0           21018 2006-07-24 02:21 qlog-0.15.2/src/events/quic.rs
-rw-r--r-- 0/0            2738 2006-07-24 02:21 qlog-0.15.2/src/events/security.rs
-rw-r--r-- 0/0           27447 2006-07-24 02:21 qlog-0.15.2/src/lib.rs
-rw-r--r-- 0/0            3700 2006-07-24 02:21 qlog-0.15.2/src/reader.rs
-rw-r--r-- 0/0           22216 2006-07-24 02:21 qlog-0.15.2/src/streamer.rs

This would fix #1271.