clean up check selection code

This new approach allows filtering out checks from the default set that are
not appropriate for a command, rather than having to list every check
that is appropriate. It also reduces some boilerplate.

Haskell does not define Eq for functions, so I had to go a long way around
with each check having a unique id. Meh.

Command/FromKey.hs

@@ -13,9 +13,8 @@ import qualified Annex.Queue
 import Annex.Content
 import Types.Key
 
-command :: [Command]
-command = [Command "fromkey" paramPath defaultChecks seek
-	"adds a file using a specific key"]
+def :: [Command]
+def = [command "fromkey" paramPath seek "adds a file using a specific key"]
 
 seek :: [CommandSeek]
 seek = [withFilesMissing start]