Fix --onlydeps flag.

Fix --onlydeps flag.

When --onlydeps is specified, we don't want to merge packages that were
specified on the command-line. Previously, it threw an exception in this
case because it couldn't find the command-line packages in the list of
installed packages.

TEST=Run ./parallel_emerge --onlydeps autotest, and make sure it
     doesn't crash.
     Run ./parallel_emerge --board=x86-generic  -po chromeos power_manager and make sure it does emerge power_manager, and doesn't crash.
BUG=none

[Nick Sanders, 2010-08-04 20:49:06]

lgtm w/ comments

http://codereview.chromium.org/2868094/diff/1/2
File parallel_emerge (right):

http://codereview.chromium.org/2868094/diff/1/2#newcode665
parallel_emerge:665: if not selective and "--onlydeps" not in emerge.opts:
Can you add a comment on functionality here?

Basically just "Take special account of packages specificed on the command line,
don't emerge them if --onlydeps (only emerge dependencies), and don't allow them
to be optional unless --selective (optional) is specified.

[Nick Sanders, 2010-08-04 21:24:21]

lgtm