ICU4X should avoid having ICU4X in its dependency tree via url crate

[sffc]

Currently we have the url crate in our dependency tree via icu_provider_source, which causes us to pull in multiple copies of icu4x:

icu_provider_source v2.0.0-beta1 (/usr/local/google/home/sffc/projects/icu4x/provider/source)
├── calendrical_calculations v0.1.2 
...
├── icu v2.0.0-beta1 (/usr/local/google/home/sffc/projects/icu4x/components/icu)
│   ├── icu_normalizer v2.0.0-beta1 (/usr/local/google/home/sffc/projects/icu4x/components/normalizer) (*)
...
├── ureq v2.10.1
│   ├── url v2.5.4
│   │   ├── idna v1.0.3
│   │   │   ├── idna_adapter v1.2.0
│   │   │   │   ├── icu_normalizer v1.5.0

We should try to avoid this. I think the url crate was considering making icu4x an optional feature. Alternatively, maybe we should use something other than ureq. That would still be brittle because any of our other dependencies could still choose at any time to add url back as a dependency.

@Manishearth @hsivonen

[sffc]

Note that this is also in our main Cargo.lock file.

[[package]]
name = "idna_adapter"
version = "1.2.0"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "daca1df1c957320b2cf139ac61e7bd64fed304c5040df000a745aa1de3b4ef71"
dependencies = [
 "icu_normalizer 1.5.0",
 "icu_properties 1.5.1",
]

[Manishearth]

It's going to be really hard to do any networking without the url crate, it's basically the only thing out there.

One of the reasons there was a much higher bar to make such changes to the crate was that there's no well integrated alternative.

We're not going to be able to drop that dep.