Remove optional dependencies if --required-only is set.

[brianhou]

Yet another proposal for dealing with optional dependencies (instead of #13, #14).

• By default, all dependencies (including optional dependencies) are required.
• Use <optional> tags (inside <export>) to mark which of the <depend>s are optional. Programmatically remove them from the set of dependencies if the --required-only flag is set in the internal-distro.py script.

[jslee02]

Looks good to me. I have one suggestion though, which shouldn't block merging this PR.

[jslee02]

The suggestion was replacing the exception with a warning. But now I see raising exception would be better because, as @brianhou pointed in a local discussion, we'll never see this error since no one runs cleanroom locally.

Merging this PR.