feat(ex_checkhealth): provide function for command line completion

Move away from providing completion with ExpandRTDir to ExpandGeneric
providing the function get_healthcheck_name which caches the results for
the current command line prompt.

It does the almost the same thing the Vim function 'get_healthcheck'
implemented in 'runtime/autoload/health.vim' does.

test/functional/plugin/health_spec.lua

@@ -36,6 +36,7 @@ describe(':checkhealth', function()
     clear()
     eq('nvim', getcompletion('nvim', 'checkhealth')[1])
     eq('provider', getcompletion('prov', 'checkhealth')[1])
+    eq('vim.lsp', getcompletion('vim.ls', 'checkhealth')[1])
   end)
 end)