Add environment variable ROS_PACKAGE_PATH to paths searched for messages

[ssnover]

This PR adds the ROS_PACKAGE_PATH as a source for paths where messages will be searched for. This is the variable used by tools like rospack which are in turn used by rosmsg and rossrv for finding packages on the system.

I noticed that this block of code is duplicated in a couple places and I think it makes sense to try to de-duplicate it, but I'll leave that to another issue/merge request.

This seeks to close issue #166

[ssnover]

@adnanademovic are you accepting contributions to this repository?

[adnanademovic]

Looks good, thanks!

I've been on a bit of a hiatus, contributions are appreciated.