Parse and format import package shorthand

The original proposal [1] suggested dropping the package shorthand,
but we later decided to keep it [2] to improve UX.

[1] https://docs.google.com/document/d/1E_77fO-44BtoBtXoVeWyGh1xN2KRTWTu8q6i25RNNx0/edit?usp=sharing
[2] 385104011

crates/compiler/parse/src/header.rs

@@ -128,10 +128,6 @@ impl<'a> ModuleName<'a> {
     pub const fn as_str(&'a self) -> &'a str {
         self.0
     }
-
-    pub fn contains_dot(&self) -> bool {
-        self.0.contains('.')
-    }
 }
 
 #[derive(Copy, Clone, PartialEq, Eq, Debug, Hash)]