Fix labels to work with canonical label literals. (bazelbuild#1700)

For more details, see
https://docs.google.com/document/d/1N81qfCa8oskCk5LqTW-LNthy6EBrDot7bdUsjz6JFC4/edit#heading=h.5mcn15i0e1ch.

This allows us to use crate_universe on a repo with bzlmod enabled.

crate_universe/src/utils/starlark/label.rs

@@ -20,7 +20,7 @@ impl FromStr for Label {
     type Err = anyhow::Error;
 
     fn from_str(s: &str) -> Result<Self, Self::Err> {
-        let re = Regex::new(r"^(@[\w\d\-_\.]*)?/{0,2}([\w\d\-_\./+]+)?(:([\+\w\d\-_\./]+))?$")?;
+        let re = Regex::new(r"^(@@?[\w\d\-_\.]*)?/{0,2}([\w\d\-_\./+]+)?(:([\+\w\d\-_\./]+))?$")?;
         let cap = re
             .captures(s)
             .with_context(|| format!("Failed to parse label from string: {}", s))?;
@@ -192,6 +192,15 @@ mod test {
     use std::fs::{create_dir_all, File};
     use tempfile::tempdir;
 
+    #[test]
+    fn full_label_bzlmod() {
+        let label = Label::from_str("@@repo//package/sub_package:target").unwrap();
+        assert_eq!(label.to_string(), "@repo//package/sub_package:target");
+        assert_eq!(label.repository.unwrap(), "repo");
+        assert_eq!(label.package.unwrap(), "package/sub_package");
+        assert_eq!(label.target, "target");
+    }
+
     #[test]
     fn full_label() {
         let label = Label::from_str("@repo//package/sub_package:target").unwrap();
@@ -276,8 +285,8 @@ mod test {
     }
 
     #[test]
-    fn invalid_double_at() {
-        assert!(Label::from_str("@@repo//pkg:target").is_err());
+    fn invalid_triple_at() {
+        assert!(Label::from_str("@@@repo//pkg:target").is_err());
     }
 
     #[test]