refactor: DRY up fallback finders

Because we might be wanting to change the behavior soon, it would be
nice to be able to change in in _one_ place instead of three.

Specifically, as per:

- wincent/command-t#393 (comment)

we might be putting something in the prompt window title to indicate
what happened.

lua/wincent/commandt/private/finders/fallback.lua

@@ -0,0 +1,11 @@
+-- SPDX-FileCopyrightText: Copyright 2022-present Greg Hurrell and contributors.
+-- SPDX-License-Identifier: BSD-2-Clause
+
+-- A lazy fallback wrapper (to avoid the cost of scanning until actually needed)
+-- around the built-in file finder.
+return function(finder, directory, options)
+  return function()
+    finder.fallback = require('wincent.commandt.private.finders.file')(directory ~= '' and directory or '.', options)
+    return finder.fallback
+  end
+end

lua/wincent/commandt/private/finders/find.lua

@@ -6,15 +6,7 @@ local ffi = require('ffi')
 -- TODO: remember cached directories
 return function(directory, options)
   local finder = {}
-  -- Use a thunk to avoid cost of fallback scanning until actually needed.
-  finder.fallback = (
-    (function(d, o)
-      return function()
-        finder.fallback = require('wincent.commandt.private.finders.file')(d ~= '' and d or '.', o)
-        return finder.fallback
-      end
-    end)(directory, options)
-  )
+  finder.fallback = require('wincent.commandt.private.finders.fallback')(finder, directory, options)
   if vim.startswith(directory, './') then
     directory = directory:sub(3, -1)
   end

lua/wincent/commandt/private/finders/git.lua

@@ -8,15 +8,7 @@ local ffi = require('ffi')
 -- TODO: remember cached directories
 return function(directory, options)
   local finder = {}
-  -- Use a thunk to avoid cost of fallback scanning until actually needed.
-  finder.fallback = (
-    (function(d, o)
-      return function()
-        finder.fallback = require('wincent.commandt.private.finders.file')(d ~= '' and d or '.', o)
-        return finder.fallback
-      end
-    end)(directory, options)
-  )
+  finder.fallback = require('wincent.commandt.private.finders.fallback')(finder, directory, options)
   if directory ~= '' then
     directory = vim.fn.shellescape(directory)
   end

lua/wincent/commandt/private/finders/rg.lua

@@ -6,15 +6,7 @@ local ffi = require('ffi')
 -- TODO: remember cached directories
 return function(directory, options)
   local finder = {}
-  -- Use a thunk to avoid cost of fallback scanning until actually needed.
-  finder.fallback = (
-    (function(d, o)
-      return function()
-        finder.fallback = require('wincent.commandt.private.finders.file')(d ~= '' and d or '.', o)
-        return finder.fallback
-      end
-    end)(directory, options)
-  )
+  finder.fallback = require('wincent.commandt.private.finders.fallback')(finder, directory, options)
   if vim.startswith(directory, './') then
     directory = directory:sub(3, -1)
   end