Support remote https:// requirements files (#1332) (#2081)

## Summary

Allow using http(s) urls for constraints and requirements files handed
to the CLI, by handling paths starting with `http://` or `https://`
differently. This allows commands for such as: `uv pip install -c
https://raw.githubusercontent.com/apache/airflow/constraints-2.8.1/constraints-3.8.txt
requests`.

closes #1332

## Test Plan

Testing install using a `constraints.txt` file hosted on github in the
airflow repository:

https://github.com/jannisko/uv/blob/fbdc2eba8e5e8fb4160baae495daff8fb48df13f/crates/uv/tests/pip_install.rs#L1440-L1484

## Advice Needed

- filesystem/http dispatch is implemented at a relatively low level (at
`crates/uv-fs/src/lib.rs#read_to_string`). Should I change some naming
here so it is obvious that the function is able to dispatch?
- I kept the CLI argument for -c and -r as a PathBuf, even though now it
is technically either a path or a url. We could either keep this as is
for now, or implement a new enum for this case? The enum could then
handle dispatch to files/http.
- Using another abstraction layer like
https://docs.rs/object_store/latest/object_store/ for the
files/urls/[s3] could work as well, though I ran into a bug during
testing which I couldn't debug

Cargo.toml

@@ -3,7 +3,7 @@ members = ["crates/*"]
 exclude = [
   "scripts",
   # Needs nightly
-  "crates/uv-trampoline"
+  "crates/uv-trampoline",
 ]
 resolver = "2"
 
@@ -22,6 +22,7 @@ anyhow = { version = "1.0.80" }
 async-compression = { version = "0.4.6" }
 async-channel = { version = "2.2.0" }
 async-trait = { version = "0.1.77" }
+async-recursion = { version = "1.0.5" }
 async_http_range_reader = { version = "0.7.0" }
 async_zip = { git = "https://github.com/charliermarsh/rs-async-zip", rev = "d76801da0943de985254fc6255c0e476b57c5836", features = ["deflate"] }
 base64 = { version = "0.21.7" }

crates/requirements-txt/Cargo.toml

@@ -15,15 +15,21 @@ workspace = true
 [dependencies]
 pep440_rs = { path = "../pep440-rs", features = ["rkyv", "serde"] }
 pep508_rs = { path = "../pep508-rs", features = ["rkyv", "serde", "non-pep508-extensions"] }
+uv-cache = { path = "../uv-cache" }
+uv-client = { path = "../uv-client" }
 uv-fs = { path = "../uv-fs" }
 uv-normalize = { path = "../uv-normalize" }
 uv-warnings = { path = "../uv-warnings" }
 
+async-recursion = { workspace = true }
 fs-err = { workspace = true }
 once_cell = { workspace = true }
 regex = { workspace = true }
+reqwest = { workspace = true }
+reqwest-middleware = { workspace = true }
 serde = { workspace = true }
 thiserror = { workspace = true }
+tokio = { workspace = true }
 tracing = { workspace = true }
 unscanny = { workspace = true }
 url = { workspace = true }