Fix issues with named fragment handling code

.changeset/nice-cheetahs-yell.md

@@ -0,0 +1,10 @@
+---
+"@apollo/query-planner": patch
+"@apollo/federation-internals": patch
+---
+
+Fix issues in code to reuse named fragments. One of the fixed issue would manifest as an assertion error with a message
+looking like `Cannot add fragment of condition X (...) to parent type Y (...)`. Another would manifest itself by
+generating an invalid subgraph fetch where a field conflicts with another version of that field that is in a reused
+named fragment.
+