Disable interface extraction if not in incremental

Previously, we always extracted the public interface, even though
there's really no need to do so when not running incremental mode.

Author: MichaelLeeDBX

mypy/build.py

@@ -1327,8 +1327,11 @@ def parse_file(self) -> None:
         self.suppressed = suppressed
         self.priorities = priorities
         self.dep_line_map = dep_line_map
-        self.interface = extract_interface(self.tree)
-        self.interface_hash = self.interface.compute_hash()
+        if self.manager.options.incremental:
+            self.interface = extract_interface(self.tree)
+            self.interface_hash = self.interface.compute_hash()
+        else:
+            self.interface_hash = ""
         self.check_blockers()
 
     def patch_parent(self) -> None: