rust packaging: non-crates.io crate

[Thomas Klausner <tk%giga.or.at@localhost>]

Hi!

I took a look at packaging texlab
(https://github.com/latex-lsp/texlab) but it uses a couple of crates
that are not on crates.io.

From the Cargo.toml file:

[dependencies]
...
citeproc = { git = "https://github.com/cormacrelf/citeproc-rs";, rev = "68c7a7c16088b5d7751641ab37299ad064cdb221" }
citeproc-db = { git = "https://github.com/cormacrelf/citeproc-rs";, rev = "68c7a7c16088b5d7751641ab37299ad064cdb221" }
citeproc-io = { git = "https://github.com/cormacrelf/citeproc-rs";, rev = "68c7a7c16088b5d7751641ab37299ad064cdb221" }
csl = { git = "https://github.com/cormacrelf/citeproc-rs";, rev = "68c7a7c16088b5d7751641ab37299ad064cdb221" }
...

Can someone please add support for this to lang/rust/cargo.mk?

Thanks,
 Thomas