runetime(vim): gf maybe hang and fail

runetime(vim): gf maybe hang and fail

* searching'**' maybe terrible slow
* searching '**/plugin/' maybe fail
* the pattern for the :packadd/:import/:colorscheme
  did not consistently check for a colon, so always use
  '\%(:\s*\)\=' before the actual ex command

While at it rename the generic name vim.vim to vimgoto.vim as this more
clearly states what this script is for.

Signed-off-by: Shane-XB-Qian <shane.qian@foxmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>