refactor: Move fs.* to util/fs.*

The fs.* files are already part of the libbitcoin_util library. With the
introduction of the fs_helpers.* it makes sense to move fs.* into the
util/ directory as well.

src/validation.h

@@ -15,7 +15,6 @@
 #include <chain.h>
 #include <consensus/amount.h>
 #include <deploymentstatus.h>
-#include <fs.h>
 #include <kernel/chainparams.h>
 #include <kernel/chainstatemanager_opts.h>
 #include <kernel/cs_main.h> // IWYU pragma: export
@@ -30,6 +29,7 @@
 #include <txmempool.h> // For CTxMemPool::cs
 #include <uint256.h>
 #include <util/check.h>
+#include <util/fs.h>
 #include <util/hasher.h>
 #include <util/translation.h>
 #include <versionbits.h>