Fix ``uv pip install -r /dev/stdin`` (#16855)

## Summary

Fix ``uv pip install -r /dev/stdin`` which was broken in uv 0.9.12 by
https://github.com/astral-sh/uv/pull/16805 .

Example of the issue:

```
$ echo "flask" | uv pip install -r /dev/stdin
warning: Requirements file `/dev/stdin` does not contain any dependencies
Audited in 8ms
```

Note that "upstream" ``pip install`` does support `-r /dev/stdin` and
doesn't support `-r -` .

## Test Plan

2 new tests added.

crates/uv-requirements/src/sources.rs

@@ -65,7 +65,7 @@ impl RequirementsSource {
             .is_some_and(|ext| ext.eq_ignore_ascii_case("txt") || ext.eq_ignore_ascii_case("in"))
         {
             Ok(Self::RequirementsTxt(path))
-        } else if path == Path::new("-") {
+        } else if path == Path::new("-") || path == Path::new("/dev/stdin") {
             // If the path is `-`, treat it as a requirements.txt file from stdin.
             Ok(Self::RequirementsTxt(path))
         } else if path.extension().is_none() {

crates/uv/tests/it/pip_install.rs

@@ -555,7 +555,7 @@ fn install_requirements_txt() -> Result<()> {
 
     context.assert_command("import flask").success();
 
-    // Install Jinja2 (which should already be installed, but shouldn't remove other packages).
+    // Install iniconfig (which shouldn't remove other packages).
     let requirements_txt = context.temp_dir.child("requirements.txt");
     requirements_txt.write_str("iniconfig")?;
 
@@ -580,6 +580,81 @@ fn install_requirements_txt() -> Result<()> {
     Ok(())
 }
 
+/// Install a package from a `requirements.txt` passed via `-r -` into a virtual environment.
+#[test]
+#[allow(clippy::disallowed_types)]
+fn install_from_stdin() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    // Install Flask.
+    let requirements_txt = context.temp_dir.child("requirements.txt");
+    requirements_txt.write_str("Flask")?;
+
+    uv_snapshot!(context.pip_install()
+        .arg("-r")
+        .arg("-")
+        .arg("--strict").stdin(std::fs::File::open(requirements_txt)?), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 7 packages in [TIME]
+    Prepared 7 packages in [TIME]
+    Installed 7 packages in [TIME]
+     + blinker==1.7.0
+     + click==8.1.7
+     + flask==3.0.2
+     + itsdangerous==2.1.2
+     + jinja2==3.1.3
+     + markupsafe==2.1.5
+     + werkzeug==3.0.1
+    "###
+    );
+
+    context.assert_command("import flask").success();
+
+    Ok(())
+}
+
+/// Install a package from a `requirements.txt` passed via `-r /dev/stdin` into a virtual environment.
+#[test]
+#[cfg(not(windows))]
+#[allow(clippy::disallowed_types)]
+fn install_from_dev_stdin() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    // Install Flask.
+    let requirements_txt = context.temp_dir.child("requirements.txt");
+    requirements_txt.write_str("Flask")?;
+
+    uv_snapshot!(context.pip_install()
+        .arg("-r")
+        .arg("/dev/stdin")
+        .arg("--strict").stdin(std::fs::File::open(requirements_txt)?), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 7 packages in [TIME]
+    Prepared 7 packages in [TIME]
+    Installed 7 packages in [TIME]
+     + blinker==1.7.0
+     + click==8.1.7
+     + flask==3.0.2
+     + itsdangerous==2.1.2
+     + jinja2==3.1.3
+     + markupsafe==2.1.5
+     + werkzeug==3.0.1
+    "###
+    );
+
+    context.assert_command("import flask").success();
+
+    Ok(())
+}
+
 /// Install a package from a remote `requirements.txt` into a virtual environment.
 #[tokio::test]
 async fn install_remote_requirements_txt() -> Result<()> {