Teach rosdep to use ROS_VERSION when resolving conditionals

[cottsay]

The metadata from REP153 includes the ROS_PYTHON_VERSION, which is currently cached during a call to rosdep update so that it is available to package manifest conditionals during rosdep operations in an environment where ROS_PYTHON_VERSION is not already set.

This change extends that metadata caching to include the ROS_VERSION variable as well. These two values combined with ROS_DISTRO, are the three variables supplied by Bloom during manifest conditional evaluation. It makes sense to treat them the same way here.

Reproduction:

1. Start with a clean environment with no workspaces sourced
2. $ git clone https://github.com/autowarefoundation/tvm_vendor.git (or any package which uses ROS_VERSION)
3. $ rosdep keys --rosdistro humble --from-path tvm_vendor

Expected results:

ocl-icd-opencl-dev
spirv-headers
libopenblas-dev
git
libxml2
opencl-headers
ament_cmake
spirv-tools
ros_environment
libvulkan-dev

Actual results:

libopenblas-dev
libxml2
ros_environment
git

[codecov-commenter]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (f3200d0) 74.28% compared to head (3356db4) 74.22%.

Files	Patch %	Lines
src/rosdep2/sources_list.py	55.55%	4 Missing ⚠️
src/rosdep2/main.py	75.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #941      +/-   ##
==========================================
- Coverage   74.28%   74.22%   -0.06%     
==========================================
  Files          44       44              
  Lines        3364     3376      +12     
==========================================
+ Hits         2499     2506       +7     
- Misses        865      870       +5     

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