update to 0.52.0, improve packaging scripts
this MR contains just the meat of the changes for evaluation. I pushed a complete run including gbp import-orig to my fork's debian/experimental, upstream and pristine-tar branches. the orig tarball is based on debcargo-conf's cargo-wip branch (87306fb573bf568b58f43b3856df88244636268d).
d/copyright needs an update still, and some of the vendored deps could probably benefit from a patch removing stuff like favicons/status icons which lintian doesn't like.
the first three commits are not related to the update, but seem like useful improvements to me.
the duplicate rand_* versions are part of this upstream release, and not caused by our patching:
$ cargo tree --duplicates
hex v0.3.2
└── crypto-hash v0.3.4
    └── cargo v0.52.0 (/tmp/cargo)
        └── cargo-test-support v0.1.0 (/tmp/cargo/crates/cargo-test-support)
            [dev-dependencies]
            └── cargo v0.52.0 (/tmp/cargo) (*)
hex v0.4.3
└── cargo v0.52.0 (/tmp/cargo) (*)
rand_core v0.5.1
├── im-rc v15.0.0
│   └── cargo v0.52.0 (/tmp/cargo) (*)
└── rand_xoshiro v0.4.0
    └── im-rc v15.0.0 (*)
rand_core v0.6.2
├── rand v0.8.3
│   └── tempfile v3.2.0
│       └── cargo v0.52.0 (/tmp/cargo) (*)
└── rand_chacha v0.3.0
    └── rand v0.8.3 (*)