Optimize upgrade of already-satisfied pinned requirement

Example: after installing six 1.12.0, `pip install -Uv six==1.12.0`
now returns immediately, instead of going to the index to check for a
version that can’t possibly be considered better.

This optimization is most significant when upgrading via a
requirements file with many pinned versions and some non-pinned
versions.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

news/7132.feature

@@ -0,0 +1 @@
+Skip reaching out to the package index, if a pinned version is already satisfied.

src/pip/_internal/legacy_resolve.py

@@ -267,6 +267,10 @@ def _check_skip_installed(self, req_to_install):
         # requirements we have to pull the tree down and inspect to assess
         # the version #, so it's handled way down.
         if not req_to_install.link:
+            if req_to_install.is_pinned:
+                # No need to check the index for a better version.
+                return 'already satisfied'
+
             try:
                 self.finder.find_requirement(req_to_install, upgrade=True)
             except BestVersionAlreadyInstalled: