* Allow to use git executable for fetching advisory database

* fix review findings

* Remove 'fix' functionality

This functionality was only added when there was the potential for
cargo-deny to supplant cargo-audit, which is no longer the case. This
functionality was quite a lot of code, minimally tested and increased
compile times (toml_edit), all for a feature that, at least
anecdotally, is not really useful as any moderately complex dependency
graph is unlikely to be patchable.

* Remove index

* Replace structopt with clap 3

* Remove rustsec git dependency

Rustsec has been really slow updating to a version with git 0.14 to
resolve a segfault, which also blocked cargo-deny. Instead, we now
handle all git fetching ourselves so we are no longer blocked, now or in
the future. This also changes it so that we use crates-index as a
dependency as previously we had included this functionality directly as
at the time crates-index didn't support bare repositories.

* Add metadata

* Remove unused dependencies

* Set MSRV to 1.60.0 due to cargo 0.61

* Deterministically order duplicate versions graphs

* Add test for duplicate ordering

* Add back MSRV check

* Update CHANGELOG

* Update docs