Refactor interface dependency resolution

Refactor interface dependency resolution

This refactors simplifies dependency resolution, and makes it easier to
extend.

Specifically:
* Move file handling into idl_reader
* Move actual definition and interface merging to methods on IdlDefinitions
  and IdlInterface (in idl_definitions)

This also clarifies exactly what's going on specially:
* we tack on some extended attributes to members
* we merge implemented interfaces to the main one

Other changes:
* use next() instead of looping over a one-element list
* remove useless exceptions (specific exception classes are only useful
  if we catch them, otherwise it's no better than a string)

This is to support a separate CL:
Bindings: inherit enums and callbacks from partial interface files.
https://codereview.chromium.org/203193015/

TBR=haraken
BUG=345137
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=169613

[Nils Barth (inactive), 2014-03-20 02:05:34]

A refactoring which will help Mounir; PTAL.
(and makes the code rather clearer anyway)

[Nils Barth (inactive), 2014-03-20 02:07:21]

The CQ bit was checked by nbarth@chromium.org

[commit-bot: I haz the power, 2014-03-20 02:07:31]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/nbarth@chromium.org/205873003/1

[commit-bot: I haz the power, 2014-03-20 04:11:38]

Change committed as 169613

[haraken, 2014-03-20 04:29:48]

LGTM