Add support for rospack_norecurse to avoid recursing into all subdirecto...

...ries when looking for a package.

I am installing ros into /usr, hence this searches in /usr/share. I have a lot of directories, not ROS related, into /usr/share (e.g. a full TeX Live install), and I get these timings:

Without rospack_norecurse in /usr/share:
$ time rosversion rospack
2.2.5

real    0m2.861s
user    0m1.588s
sys     0m1.267s

With rospack_norecurse in /usr/share:
$ time rosversion rospack
2.2.5

real    0m0.135s
user    0m0.100s
sys     0m0.034s

This is on a desktop with a SSD drive. On an arm board, with much less packages in /usr/share but way much slower I/O I remember hitting the 10 seconds timeout for roscore to start.
Yes, I "invented" rospack_norecurse mimicking rospack_nosubdirs; feel free to discuss it and put this PR on hold meanwhile: if there is a better solution you'd know much better than me anyway. The only thing I have to say is that scanning all the subfolders is extremely slow when ros is installed as a system package (prefix is /usr, binaries end up in /usr/bin, etc.).

As for how it is created, currently I was using "touch" or installing it with my rospkg package :)