feat: add installer to InstalledDist (#1762)

## Summary

Add `installer` method to `InstalledDist` to distinguish between
different installers. Might be nice to add an enum for all possible
installers, but this might be too hard to keep up to date :).

The `INSTALLER` file is a file that can be added to the `.dist-info`
folder with the installer name.

Closes: #1759 

## Test Plan

Not sure if there is a place I can automatically test it, if you have a
pointer I would be happy to add a test.

crates/distribution-types/src/installed.rs

@@ -111,6 +111,16 @@ impl InstalledDist {
         })
     }
 
+    /// Return the `INSTALLER` of the distribution.
+    pub fn installer(&self) -> Result<Option<String>> {
+        let path = self.path().join("INSTALLER");
+        match fs::read_to_string(path) {
+            Ok(installer) => Ok(Some(installer)),
+            Err(err) if err.kind() == std::io::ErrorKind::NotFound => Ok(None),
+            Err(err) => Err(err.into()),
+        }
+    }
+
     /// Return the [`Url`] of the distribution, if it is editable.
     pub fn as_editable(&self) -> Option<&Url> {
         match self {