validation: Move LookupBlockIndex to BlockManager

[META] This commit should be followed up by a scripted-diff commit which
       fixes calls to LookupBlockIndex tree-wide.
[META] This commit should be followed up by removing the comments and
       assertions meant only to show that the change is correct.

LookupBlockIndex only acts on BlockManager.

src/validation.h

@@ -433,6 +433,8 @@ public:
         const CChainParams& chainparams,
         CBlockIndex** ppindex) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
+    CBlockIndex* LookupBlockIndex(const uint256& hash) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
+
     ~BlockManager() {
         Unload();
     }