counsel.el: Strip space from initial input of counsel-package

[iquiw]

This could reduce key presses in most cases.

Filter packages to ...	# of key presses with ^+ (space)	# of key presses with ^+ (no space)
install package starting with x	2	1
uninstall package starting with x	4	3
install package containing x	1	2
uninstall package containing x	5	4

For example,

• If initial input is ^+ (space), it requires Backspace Backspace - x to filter packages to uninstall package starting with x
• If initial input is ^+ (no space), it requires Backspace - x to filter packages to uninstall package starting with x

I'd like to do this small change before #1987.

[abo-abo]

Thanks.