Move ExcludeNewer into its own type (#3041)

## Summary

This makes it easier to add (e.g.) JSON Schema derivations to the type.

If we have support for other dates in the future, we can generalize it
to a `UserDate` or similar.

crates/uv-resolver/src/exclude_newer.rs

@@ -0,0 +1,53 @@
+use std::str::FromStr;
+
+use chrono::{DateTime, Days, NaiveDate, NaiveTime, Utc};
+
+/// A timestamp that excludes files newer than it.
+#[derive(Debug, Copy, Clone)]
+pub struct ExcludeNewer(DateTime<Utc>);
+
+impl ExcludeNewer {
+    /// Returns the timestamp in milliseconds.
+    pub fn timestamp_millis(&self) -> i64 {
+        self.0.timestamp_millis()
+    }
+}
+
+impl From<DateTime<Utc>> for ExcludeNewer {
+    fn from(datetime: DateTime<Utc>) -> Self {
+        Self(datetime)
+    }
+}
+
+impl FromStr for ExcludeNewer {
+    type Err = String;
+
+    /// Parse an [`ExcludeNewer`] from a string.
+    ///
+    /// Accepts both RFC 3339 timestamps (e.g., `2006-12-02T02:07:43Z`) and UTC dates in the same
+    /// format (e.g., `2006-12-02`).
+    fn from_str(input: &str) -> Result<Self, Self::Err> {
+        let date_err = match NaiveDate::from_str(input) {
+            Ok(date) => {
+                // Midnight that day is 00:00:00 the next day
+                return Ok(Self(
+                    (date + Days::new(1)).and_time(NaiveTime::MIN).and_utc(),
+                ));
+            }
+            Err(err) => err,
+        };
+        let datetime_err = match DateTime::parse_from_rfc3339(input) {
+            Ok(datetime) => return Ok(Self(datetime.with_timezone(&Utc))),
+            Err(err) => err,
+        };
+        Err(format!(
+            "`{input}` is neither a valid date ({date_err}) nor a valid datetime ({datetime_err})"
+        ))
+    }
+}
+
+impl std::fmt::Display for ExcludeNewer {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        self.0.fmt(f)
+    }
+}