[Windows][melodic] remove path splash separator from 'package_dir'

[seanyen]

When running catkin_make (or catkin_make_isolated) on Windows, distutils throws errors for cv_bridge. And it turned out that distutils doesn't like the path separator not matched to the underlying OS convention in setup.py.

Traceback (most recent call last):
  File "C:/dr_ws/src/vision_opencv/cv_bridge\setup.py", line 11, in <module>
    setup(**d)
  File "C:\opt\python27amd64\lib\distutils\core.py", line 151, in setup
    dist.run_commands()
  File "C:\opt\python27amd64\lib\distutils\dist.py", line 953, in run_commands
    self.run_command(cmd)
  File "C:\opt\python27amd64\lib\distutils\dist.py", line 972, in run_command
    cmd_obj.run()
  File "C:\opt\python27amd64\lib\distutils\command\build.py", line 127, in run
    self.run_command(cmd_name)
  File "C:\opt\python27amd64\lib\distutils\cmd.py", line 326, in run_command
    self.distribution.run_command(command)
  File "C:\opt\python27amd64\lib\distutils\dist.py", line 971, in run_command
    cmd_obj.ensure_finalized()
  File "C:\opt\python27amd64\lib\distutils\cmd.py", line 109, in ensure_finalized
    self.finalize_options()
  File "C:\opt\python27amd64\lib\distutils\command\build_py.py", line 56, in finalize_options
    self.package_dir[name] = convert_path(path)
  File "C:\opt\python27amd64\lib\distutils\util.py", line 126, in convert_path
    raise ValueError, "path '%s' cannot end with '/'" % pathname
ValueError: path 'python/' cannot end with '/'

Per the example in catkin doc, it is not mandatory to tail the package_dir with / so it can be easily fixed by removing it and make the code more portable.

[mjcarroll]

@seanyen Can you give this a quick rebase on the latest melodic branch to see that it works with travis?

[seanyen]

@mjcarroll I just did a rebase and it seemed the CI result looking good.

[mjcarroll]

Thanks for the contribution!