Make .dist lazily resolver again

Preparing a distribution also fetches it if it's not already installed.
We only want to do it when we absolutely need it.

news/9284.bugfix.rst

@@ -0,0 +1 @@
+New resolver: Only download distributions when we really need to inspect them.

src/pip/_internal/resolution/resolvelib/candidates.py

@@ -141,7 +141,7 @@ def __init__(
         self._ireq = ireq
         self._name = name
         self._version = version
-        self.dist = self._prepare()
+        self._dist = None  # type: Optional[Distribution]
 
     def __str__(self):
         # type: () -> str
@@ -169,6 +169,14 @@ def __ne__(self, other):
         # type: (Any) -> bool
         return not self.__eq__(other)
 
+    @property
+    def dist(self):
+        # type: () -> Distribution
+        if self._dist is not None:
+            return self._dist
+        self._dist = dist = self._prepare()
+        return dist
+
     @property
     def source_link(self):
         # type: () -> Optional[Link]