IDL compiler: Simplify dependency resolution

This removes no longer needed checks, since we're only compiling
actual IDL files. This lets us remove a small function.
Also:
* flatten logic
* improve documentation
* remove special case for exceptions (not needed)
* fix naming ('member' of an interface, not 'element')

Eliminate BindingsDerivedSources.txt
https://codereview.chromium.org/135903003/

TBR=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/144633003

git-svn-id: svn://svn.chromium.org/blink/trunk@165505 bbb929c8-8fbe-4397-9dbb-9b2b20218538