New roclets for depends and index and alterations to update Imports field

Hi Hadley, I sent you these changes some time ago by email and you asked me to perform a pull request.
I have finally got around to learning git so here is my pull request.
The changes I made seem to work fine for me, but I get the following error for one of your tests when I run the "check" command of the devtools package:
 
  Running ‘test-all.R’
 ERROR
Running the tests in ‘tests/test-all.R’ failed.
Last 13 lines of output:
  Rd - examples : ........
  Rd - usage : ......1..
  Rd : ....................................................
  Wrap DESCRIPTION fields only when necessary : .........
  
  
  1. Failure: @usage overrides default for @docType data -------------------------
  get_tag(out, "usage")$values not equal to "data(abc)"
  Modes: character, NULL
  Lengths: 1, 0
  target is character, current is NULL