Rename GN source_prereqs to inputs, enhance "desc"

Rename GN source_prereqs to inputs, enhance "desc"

We picked the name "source_prereqs" to try to emphasize that these are source
(compile-time) dependencies rather than data or link-time dependencies.
However, now that all actions are implicit hard deps, the use-case of using
this for binary targets is almost nonexistant (I want to eventually put back
actions-as-soft-deps but this requires some additional checking to make sure
people don't mess up, at which time this may becomre more relevant.)

source_prereqs is hard to type and difficult to understand. Inputs is much more
clear, especially since the typical case is an action.

This will fall back to reading "source_prereqs" for back-compat until we change
all users.

Significantly enhance the "gn desc" command. A bunch of action-related fields
were missing.

BUG=374271
R=jamesr@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=277850

[jamesr, 2014-06-16 20:18:04]

lgtm

[brettw, 2014-06-17 20:26:59]

Committed patchset #2 manually as r277850 (presubmit successful).