Reducing character count for path to generated messages

The path from the package root to the root of the PregeneratedMessages
folder was 47 characters. From within the PickAndPlace project, the
fully qualified path including drive letter, path to workspace, and path
to its location in the package cache can easily exceed 200 characters,
especially for users with long names or deep workspace organization.
The maximum allowed path length for a fully qualified path in Windows is
260 characters, after which an IDE can not compile the assemblies for
source with too-long paths.

Moved Runtime/MessageGeneration/PregeneratedMessages to
Runtime/Messages, which saves 30 characters off the full path name. This
is a mitigation that can not fully solve the problem, but wil at least
reduce the number of cases for which the ROS-TCP-Endpoint assemblies
fail to compile due to path length.

Tested on my machine by manually renaming the directory in my
PackageCache.