message: Remove some dead code in SourceMap
diff: @@ -264,11 +264,6 @@ impl<'src> SourceMap<'src> {
  }
  }
  - /// Like `HashMap::contains_key`.
  - pub fn contains(&self, id: SourceId) -> bool {
  - self.map.contains_key(&id)
  - }
  -
  /// Like `HashMap::get`.
  pub fn get(&self, id: SourceId) -> Option<&(dyn Source + 'src)> {
  self.map.get(&id).map(|s| s.as_ref())
  @@ -279,32 +274,17 @@ impl<'src> SourceMap<'src> {
  self.map.get_mut(&id).map(|s| s.as_mut())
  }
  - /// Like `HashMap::get`, but first calculates the `SourceId` from a `PackageId`.
  - pub fn get_by_package_id(&self, pkg_id: PackageId) -> Option<&(dyn Source + 'src)> {
  - self.get(pkg_id.source_id())
  - }
  -
  /// Like `HashMap::insert`, but derives the `SourceId` key from the `Source`.
  pub fn insert(&mut self, source: Box<dyn Source + 'src>) {
  let id = source.source_id();
  self.map.insert(id, source);
  }
  - /// Like `HashMap::is_empty`.
  - pub fn is_empty(&self) -> bool {
  - self.map.is_empty()
  - }
  -
  /// Like `HashMap::len`.
  pub fn len(&self) -> usize {
  self.map.len()
  }
  - /// Like `HashMap::values`.
  - pub fn sources<'a>(&'a self) -> impl Iterator<Item = &'a Box<dyn Source + 'src>> {
  - self.map.values()
  - }
  -
  /// Like `HashMap::iter_mut`.
  pub fn sources_mut<'a>(
  &'a mut self,
  