Fix get_depends returning 1st run) none then on 2nd run) fewer pkgs

[130s]

Issue: As described in #142.

Suggested solution

• Add a partial fix in this PR.
  • On 1st run, return all dependencies that are already captured by the time traversing dependency terminates (w/o this PR, no dependency returns).
  • On 2nd run, return all dependencies that are supposed to be returned (w/o this PR, only part of dependency returns).
• To fully fix, Fix inconsistent return value of get_rosdeps #141 is also needed (but following the discussion in Fix get_{depends, rosdeps} error when depended pkgs are not available in the environment #129, I split PRs).

Note that I'm not adding a test case for this specific issue. To reproduce this issue requires traversing the dependency chain AFAICT, but looks like CI environment is kept plain so no ros pkgs are installed thus hard to replicate the environment. get_depends is tested in multiple test cases (e.g. test_rospkg_packages.py).

Output

• Without this PR

See the output in #142 (comment)

• With this PR

root@d5a64a312993:~# SRCDIR=~/cws/src && mkdir -p $SRCDIR && git clone https://github.com/plusone-robotics/rospkg -b fix_get_depends_issue142
root@d5a64a312993:~/cws/src/rospkg# export PYTHONPATH=$SRCDIR/rospkg/src:$PYTHONPATH
root@d5a64a312993:~/cws/src/rospkg# export PATH=$SRCDIR/rospkg/scripts:$PATH
root@d5a64a312993:~/cws/src/rospkg# ipython

In [1]: import rospkg
In [2]: rp = rospkg.RosPack()
In [3]: rp.get_depends("roscpp")
---------------------------------------------------------------------------
ResourceNotFound                          Traceback (most recent call last)
<ipython-input-3-b793dd583b6b> in <module>()
----> 1 rp.get_depends("roscpp")

/root/cws/src/rospkg/src/rospkg/rospack.py in get_depends(self, name, implicit)
    267                     ros_paths=self._ros_paths,
        268                     deps_sofar=s,
--> 269                     deps_unavailable=depends_unavailable)
    270             return s
        271

ResourceNotFound: Pkg(s) ['roslang', 'rosunit'] not available on your environment.
Defined dependency can be obtained in ResourceNotFound.get_depends: ['rosgraph_msgs', 'genlisp', 'genpy', 'genmsg', 'catkin', 'rosconsole', 'gencpp', 'xmlrpcpp', 'rostime', 'roslang', 'message_runtime', 'std_msgs', 'geneus', 'gennodejs', 'roscpp_traits', 'roscpp_serialization', 'rosbuild', 'message_generation', 'cpp_common', 'rosunit']
ROS path [0]=/opt/ros/melodic/share/ros
ROS path [1]=/opt/ros/melodic/share

In [4]: rp.get_depends("roscpp")
Out[4]:
['rosgraph_msgs',
 'genlisp',
 'genpy',
 'genmsg',
 'catkin',
 'rosconsole',
 'gencpp',
 'xmlrpcpp',
 'rostime',
 'roslang',
 'message_runtime',
 'std_msgs',
 'geneus',
 'gennodejs',
 'roscpp_traits',
 'roscpp_serialization',
 'rosbuild',
 'message_generation',
 'cpp_common',
 'rosunit']

[130s]

Please review.

[130s]

Please review. I'd call #142 a bug, and this PR is meant to amend the situation about it.

[dirk-thomas]

Blocked by #141 due to the significant overlap of the patch.

[dirk-thomas]

Due to the long time of inactivity I will go ahead and close the ticket. Please feel free to comment with updates and it can be reopened or feel free to open a new PR when you are ready referencing this one.