add ExportTree table to export db

New table needed to look up what filenames are used in the currently
exported tree, for reasons explained in export.mdwn.

Also, added smart constructors for ExportLocation and ExportDirectory to
make sure they contain filepaths with the right direction slashes.

And some code refactoring.

This commit was sponsored by Francois Marier on Patreon.

Remote/Helper/Export.hs

@@ -93,7 +93,7 @@ adjustExportable r = case M.lookup "exporttree" (config r) of
 			-- Storing a key on an export would need a way to
 			-- look up the file(s) that the currently exported
 			-- tree uses for a key; there's not currently an
-			-- inexpensive way to do that (getExportLocation
+			-- inexpensive way to do that (getExportedLocation
 			-- only finds files that have been stored on the
 			-- export already).
 			{ storeKey = \_ _ _ -> do
@@ -105,7 +105,7 @@ adjustExportable r = case M.lookup "exporttree" (config r) of
 			, retrieveKeyFile = \k _af dest p -> unVerified $
 				if maybe False (isJust . verifyKeyContent) (maybeLookupBackendVariety (keyVariety k))
 					then do
-						locs <- liftIO $ getExportLocation db k
+						locs <- liftIO $ getExportedLocation db k
 						case locs of
 							[] -> do
 								warning "unknown export location"
@@ -136,7 +136,7 @@ adjustExportable r = case M.lookup "exporttree" (config r) of
 			, checkPresent = \k -> do
 				ea <- exportActions r
 				anyM (checkPresentExport ea k)
-					=<< liftIO (getExportLocation db k)
+					=<< liftIO (getExportedLocation db k)
 			, mkUnavailable = return Nothing
 			, getInfo = do
 				is <- getInfo r
@@ -155,10 +155,10 @@ removeEmptyDirectories ea db loc ks
 			ok <- allM (go removeexportdirectory) 
 				(reverse (exportDirectories loc))
 			unless ok $ liftIO $ do
-				-- Add back to export database, so this is
-				-- tried again next time.
+				-- Add location back to export database, 
+				-- so this is tried again next time.
 				forM_ ks $ \k ->
-					addExportLocation db k loc
+					addExportedLocation db k loc
 				flushDbQueue db
 			return ok
   where