Create a file ~/.textadept/modules/common/init.lua with the following in it:

local lfs = require 'lfs'
for filename in lfs.dir(_USERHOME..'/modules/common/') do
  if filename:find('%.lua$') and filename ~= 'init.lua' then
    require('common.'..filename:match('^(.+)%.lua$'))
  end
end

Then add

require 'common'

to your ~/.textadept/init.lua

All "common" submodules that are placed in modules/common will be automatically loaded without having to add another require statement.