scripted-diff: Use BlockManager::LookupBlockIndex

[META] In a previous commit, we moved ::LookupBlockIndex to become a
       member function of BlockManager. This commit is split out from
       that one since it can be expressed nicely as a scripted-diff.

-BEGIN VERIFY SCRIPT-
find_regex='LookupBlockIndex' \
    && git grep -l -E "$find_regex" -- src \
        | grep -v '^src/validation\.\(cpp\|h\)$' \
        | xargs sed -i -E "s@${find_regex}@g_chainman.m_blockman.LookupBlockIndex@g"
-END VERIFY SCRIPT-

src/index/base.cpp

@@ -239,7 +239,7 @@ void BaseIndex::ChainStateFlushed(const CBlockLocator& locator)
     const CBlockIndex* locator_tip_index;
     {
         LOCK(cs_main);
-        locator_tip_index = LookupBlockIndex(locator_tip_hash);
+        locator_tip_index = g_chainman.m_blockman.LookupBlockIndex(locator_tip_hash);
     }
 
     if (!locator_tip_index) {