Avoid hard-error for non-existent extras (#627)

## Summary

When resolving `transformers[tensorboard]`, the `[tensorboard]` extra
doesn't exist. Previously, we returned "unknown" dependencies for this
variant, which leads the resolution to try all versions, then fail. This
PR instead warns, but returns the base dependencies for the package,
which matches `pip`. (Poetry doesn't even warn, it just proceeds as
normal.)

Arguably, it would be better to return a custom incompatibility here and
then propagate... But this PR is better than the status quo, and I don't
know if we have support for that behavior yet...? (\cc @zanieb)

Closes #386.

Closes https://github.com/astral-sh/puffin/issues/423.

crates/puffin-cli/tests/pip_compile.rs

@@ -1720,3 +1720,103 @@ fn override_multi_dependency() -> Result<()> {
 
     Ok(())
 }
+
+/// Request an extra that doesn't exist on the specified package.
+#[test]
+fn missing_registry_extra() -> Result<()> {
+    let temp_dir = TempDir::new()?;
+    let cache_dir = TempDir::new()?;
+    let venv = create_venv_py312(&temp_dir, &cache_dir);
+
+    let requirements_in = temp_dir.child("requirements.in");
+    requirements_in.write_str("black[tensorboard]==23.10.1")?;
+
+    insta::with_settings!({
+        filters => INSTA_FILTERS.to_vec()
+    }, {
+        assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
+            .arg("pip-compile")
+            .arg("requirements.in")
+            .arg("--cache-dir")
+            .arg(cache_dir.path())
+            .arg("--exclude-newer")
+            .arg(EXCLUDE_NEWER)
+            .env("VIRTUAL_ENV", venv.as_os_str())
+            .current_dir(&temp_dir), @r###"
+        success: true
+        exit_code: 0
+        ----- stdout -----
+        # This file was autogenerated by Puffin v0.0.1 via the following command:
+        #    puffin pip-compile requirements.in --cache-dir [CACHE_DIR]
+        black==23.10.1
+        click==8.1.7
+            # via black
+        mypy-extensions==1.0.0
+            # via black
+        packaging==23.2
+            # via black
+        pathspec==0.11.2
+            # via black
+        platformdirs==4.0.0
+            # via black
+
+        ----- stderr -----
+        Resolved 6 packages in [TIME]
+        warning: The package `black==23.10.1` does not have an extra named `tensorboard`.
+        "###);
+    });
+
+    Ok(())
+}
+
+/// Request an extra that doesn't exist on the specified package.
+#[test]
+fn missing_url_extra() -> Result<()> {
+    let temp_dir = TempDir::new()?;
+    let cache_dir = TempDir::new()?;
+    let venv = create_venv_py312(&temp_dir, &cache_dir);
+
+    let requirements_in = temp_dir.child("requirements.in");
+    requirements_in.write_str("flask[tensorboard] @ https://files.pythonhosted.org/packages/36/42/015c23096649b908c809c69388a805a571a3bea44362fe87e33fc3afa01f/flask-3.0.0-py3-none-any.whl")?;
+
+    insta::with_settings!({
+        filters => INSTA_FILTERS.to_vec()
+    }, {
+        assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
+            .arg("pip-compile")
+            .arg("requirements.in")
+            .arg("--cache-dir")
+            .arg(cache_dir.path())
+            .arg("--exclude-newer")
+            .arg(EXCLUDE_NEWER)
+            .env("VIRTUAL_ENV", venv.as_os_str())
+            .current_dir(&temp_dir), @r###"
+        success: true
+        exit_code: 0
+        ----- stdout -----
+        # This file was autogenerated by Puffin v0.0.1 via the following command:
+        #    puffin pip-compile requirements.in --cache-dir [CACHE_DIR]
+        blinker==1.7.0
+            # via flask
+        click==8.1.7
+            # via flask
+        flask @ https://files.pythonhosted.org/packages/36/42/015c23096649b908c809c69388a805a571a3bea44362fe87e33fc3afa01f/flask-3.0.0-py3-none-any.whl
+        itsdangerous==2.1.2
+            # via flask
+        jinja2==3.1.2
+            # via flask
+        markupsafe==2.1.3
+            # via
+            #   jinja2
+            #   werkzeug
+        werkzeug==3.0.1
+            # via flask
+
+        ----- stderr -----
+        Resolved 7 packages in [TIME]
+        warning: The package `flask @ https://files.pythonhosted.org/packages/36/42/015c23096649b908c809c69388a805a571a3bea44362fe87e33fc3afa01f/flask-3.0.0-py3-none-any.whl` does not have an extra named `tensorboard`.
+        "###);
+    });
+
+    Ok(())
+}