Improve static metadata extraction for Poetry projects (#4182)

## Summary

Adds handling for a few cases to improve interoperability with Poetry:

- If the `project` schema is invalid, we now raise a hard error, rather
than treating the metadata as dynamic and then falling back to the build
backend. This could cause problems, I'm not sure. It's stricter than
before.
- If the project contains `tool.poetry` but omits
`project.dependencies`, we now treat it as dynamic. We could go even
further and treat _any_ Poetry project as dynamic, but then we'd be
ignoring user-declared dependencies, which is also confusing.

Closes https://github.com/astral-sh/uv/issues/4142.

crates/pypi-types/src/metadata.rs

@@ -60,6 +60,8 @@ pub enum MetadataError {
     UnsupportedMetadataVersion(String),
     #[error("The following field was marked as dynamic: {0}")]
     DynamicField(&'static str),
+    #[error("The project uses Poetry's syntax to declare its dependencies, despite including a `project` table in `pyproject.toml`")]
+    PoetrySyntax,
 }
 
 impl From<Pep508Error<VerbatimParsedUrl>> for MetadataError {
@@ -210,6 +212,15 @@ impl Metadata23 {
             }
         }
 
+        // If dependencies are declared with Poetry, and `project.dependencies` is omitted, treat
+        // the dependencies as dynamic. The inclusion of a `project` table without defining
+        // `project.dependencies` is almost certainly an error.
+        if project.dependencies.is_none()
+            && pyproject_toml.tool.and_then(|tool| tool.poetry).is_some()
+        {
+            return Err(MetadataError::PoetrySyntax);
+        }
+
         let name = project.name;
         let version = project
             .version
@@ -257,11 +268,11 @@ impl Metadata23 {
 }
 
 /// A `pyproject.toml` as specified in PEP 517.
-#[derive(Deserialize, Debug, Clone)]
+#[derive(Deserialize, Debug)]
 #[serde(rename_all = "kebab-case")]
 struct PyProjectToml {
-    /// Project metadata
     project: Option<Project>,
+    tool: Option<Tool>,
 }
 
 /// PEP 621 project metadata.
@@ -270,7 +281,7 @@ struct PyProjectToml {
 /// relevant for dependency resolution.
 ///
 /// See <https://packaging.python.org/en/latest/specifications/pyproject-toml>.
-#[derive(Deserialize, Debug, Clone)]
+#[derive(Deserialize, Debug)]
 #[serde(rename_all = "kebab-case")]
 struct Project {
     /// The name of the project
@@ -288,6 +299,17 @@ struct Project {
     dynamic: Option<Vec<String>>,
 }
 
+#[derive(Deserialize, Debug)]
+#[serde(rename_all = "kebab-case")]
+struct Tool {
+    poetry: Option<ToolPoetry>,
+}
+
+#[derive(Deserialize, Debug)]
+#[serde(rename_all = "kebab-case")]
+#[allow(clippy::empty_structs_with_brackets)]
+struct ToolPoetry {}
+
 /// Python Package Metadata 1.0 and later as specified in
 /// <https://peps.python.org/pep-0241/>.
 ///
@@ -369,6 +391,15 @@ impl RequiresDist {
             }
         }
 
+        // If dependencies are declared with Poetry, and `project.dependencies` is omitted, treat
+        // the dependencies as dynamic. The inclusion of a `project` table without defining
+        // `project.dependencies` is almost certainly an error.
+        if project.dependencies.is_none()
+            && pyproject_toml.tool.and_then(|tool| tool.poetry).is_some()
+        {
+            return Err(MetadataError::PoetrySyntax);
+        }
+
         let name = project.name;
 
         // Extract the requirements.