prep for fixing find --branch --unlocked

Added LinkType to ProvidedInfo, and unified MatchingKey with
ProvidedInfo. They're both used in the same way, so there was no real
reason to keep separate.

Note that addLocked and addUnlocked still set matchNeedsFileName,
because to handle MatchingFile, they do need it. However, they
don't use it when MatchingInfo is provided. This should be ok,
the --branch case will be able skip checking matchNeedsFileName,
since it will provide a filename in any case.

Types/Link.hs

@@ -0,0 +1,13 @@
+{- Types for links to content
+ -
+ - Copyright 2013-2021 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU AGPL version 3 or higher.
+ -}
+
+module Types.Link where
+
+-- A locked link is stored in git as a symlink, while an unlocked link is
+-- stored as a pointer file.
+data LinkType = LockedLink | UnlockedLink
+	deriving (Show, Eq)