[get_depends] when a dependency is missing, 1st run returns no pkgs, but 2nd run returns some pkgs (re-open #142).

[130s]

#142 was closed but I confirmed I still see the same issue happens.

Problem

See #142 (comment) for the detail of the issue.

Potential impact

• As seen in [get_depends] when a dependency is missing, 1st run returns no pkgs, but 2nd run returns some pkgs (only some). #142, in a few repositories there were a reference to that ticket as a potential root cause.
• That said criticality is unknown?

Related log

I re-ran the repro scenario

docker run -it ros:melodic-ros-core /bin/bash
root@427239906dca:/# apt-get update && apt-get install -y ipython3
root@427239906dca:/# apt-get purge ros-melodic-roslang && apt-get autoremove
      Reading package lists... Done
      Building dependency tree
      Reading state information... Done
      The following packages were automatically installed and are no longer required:
:
Use 'apt autoremove' to remove them.
The following packages will be REMOVED:
  ros-melodic-ros* ros-melodic-ros-comm* ros-melodic-ros-core* ros-melodic-roslang* ros-melodic-roslisp*
  0 upgraded, 0 newly installed, 5 to remove and 7 not upgraded.
  After this operation, 647 kB disk space will be freed.
  Do you want to continue? [Y/n]
:
The following packages will be REMOVED:
:
0 upgraded, 0 newly installed, 363 to remove and 4 not upgraded.

:
root@427239906dca:/# rospack profile && rospack find roslang
:
[rospack] Error: package 'roslang' not found

root@427239906dca:/# rospack find roscpp
/opt/ros/melodic/share/roscpp

root@427239906dca:/# apt install -y python-rosdep && rosdep init && rosdep update
root@427239906dca:/# apt install -y ipython && 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")

/usr/lib/python2.7/dist-packages/rospkg/rospack.pyc in get_depends(self, name, implicit)
    242
        243             for p in names:
--> 244                 s.update(self.get_depends(p, implicit))
    245             # add in our own deps
        246             s.update(names)

/usr/lib/python2.7/dist-packages/rospkg/rospack.pyc in get_depends(self, name, implicit)
    242
        243             for p in names:
--> 244                 s.update(self.get_depends(p, implicit))
    245             # add in our own deps
        246             s.update(names)

/usr/lib/python2.7/dist-packages/rospkg/rospack.pyc in get_depends(self, name, implicit)
    236
        237             # take the union of all dependencies
--> 238             names = [p.name for p in self.get_manifest(name).depends]
    239
        240             # assign key before recursive call to prevent infinite case

/usr/lib/python2.7/dist-packages/rospkg/rospack.pyc in get_manifest(self, name)
    169             return self._manifests[name]
        170         else:
--> 171             return self._load_manifest(name)
    172
        173     def _update_location_cache(self):

/usr/lib/python2.7/dist-packages/rospkg/rospack.pyc in _load_manifest(self, name)
    213         :raises: :exc:`ResourceNotFound`
        214         """
--> 215         retval = self._manifests[name] = parse_manifest_file(self.get_path(name), self._manifest_name, rospack=self)
    216         return retval
    217

/usr/lib/python2.7/dist-packages/rospkg/rospack.pyc in get_path(self, name)
    205         self._update_location_cache()
    206         if name not in self._location_cache:
--> 207             raise ResourceNotFound(name, ros_paths=self._ros_paths)
    208         else:
    209             return self._location_cache[name]

ResourceNotFound: rosunit
ROS path [0]=/opt/ros/melodic/share/ros
ROS path [1]=/opt/ros/melodic/share

In [4]: rp.get_depends("roscpp")
Out[4]: 
{'catkin',
 'cpp_common',
 'gencpp',
 'geneus',
 'genlisp',
 'genmsg',
 'gennodejs',
 'genpy',
 'message_generation',
 'message_runtime',
 'roscpp_serialization',
 'roscpp_traits',
 'rostime',
 'std_msgs'}

root@427239906dca:/# apt-cache policy python-rosdep
python-rosdep:
  Installed: 0.20.0-1
  Candidate: 0.20.0-1
  Version table:
 *** 0.20.0-1 500
        500 http://packages.ros.org/ros/ubuntu bionic/main amd64 Packages
        100 /var/lib/dpkg/status