Split generate_global_constructors.py out of compute_dependencies.py

Split generate_global_constructors.py out of compute_dependencies.py

(Long CL b/c refactoring a complex and messy script.)

Currently compute_dependencies.py is very complex, doing 3 tasks:
1. Computing interfaces info (main function)
2. Computing global constructors
3. Computing event interfaces

This splits out 2 (global constructors); event interfaces will be a followup.

The complexity makes the code hard to understand and edit (previous CLs have
required extensive review), and makes code reuse hard (e.g., in
run-bindings-tests we need to use dummy files b/c we don't care about global
constructors).

Most seriously, this makes it hard to use compute_dependencies to compute
dependencies at gyp time (needed to avoid full rebuilds), since it generates
excess files that we only want to generate at build time.

This also:
* adds documentation,
* renames compute_dependencies to compute_interfaces_info,
　as it does a lot more than just compute dependencies now
* makes the call to generate_global_constructors generic
　(so we don't need to add a flag for each global scope!), and
* factors out some common file-handling and regex IDL parsing functions.

2 followup CLs:
* Split out EventInterfaces.in
* Remove [NoInterfaceObject] from tests and dependencies:
no longer needed, since we now just don't feed these into
generate_global_constructors!

BUG=341748
R=haraken

[Nils Barth (inactive), 2014-02-21 12:02:57]

CL 1/2 of "factor compute_dependencies"
(so we can use it in GYP).
It's pretty long, since it's splitting a big file into pieces,
but content is simple.

[Nils Barth (inactive), 2014-02-21 12:32:00]

Splitting up into 3 CLs, per discussion:
1. Rename file
2. Split (no logic changes)
3. Logic changes (post-split cleanup)

[haraken, 2014-02-21 12:32:30]

On 2014/02/21 12:32:00, Nils Barth wrote:
> Splitting up into 3 CLs, per discussion:
> 1. Rename file
> 2. Split (no logic changes)
> 3. Logic changes (post-split cleanup)

Yeah, that is really helpful. Thanks!