validation: Prune (in)direct g_chainman usage related to ::LookupBlockIndex (bundle 1)

This PR builds on the work of #19927 towards #20049 (closed)

Please see #20049 (closed) for the broader context, especially this section which applies specifically to this PR:

...for some functions (LookupBlockIndex being the main culprit), a resolution of their dependency on the g_chainman/::Chain(state|)Active() globals results in an increase in apparent references to g_chainman/::Chain(state|)Active(). However, it is important to note that these are not new dependencies by any means, those dependency were always there -- they was just hidden by Qux's use of the globals. Furthermore, all of these "new" apparent references to g_chainman/::Chain(state|)Active() will be dealt with as we prune the call graph upward.

Please also read the note in the commit message of 407faf2ff364cc778083c145a77b0f150232121d (validation: Move FindForkInGlobalIndex to BlockManager), and let me know your thoughts.