This is a partial backport of core#25254
The other unrelated changes related to includes are debatable and can be done in a separate diff if we want them. Also the iwyu tool used by Core to lint these includes seems very finicky, I'm not sure it is worth the effort to port it for now.
I just checked that all files using DEFAULT_MIN_RELAY_TX_FEE include policy/policy.h and every file using minrelaytxfee includes policy/settings.h