Separate gathering rosdeps from sysroot creation

[emersonknapp]

Preserves the Docker cache on source changes.

• Create a reusable Docker image that has rosdep installed. Thought about just using rosdep on the local system as a python dependency, but this avoids having to run rosdep init as root, and avoids the potential issue of python-installed and apt-installed rosdep overlays.
• Runs rosdep against a mounted workspace to output a script for installing dependencies. If the script doesn't change between runs (meaning no dependencies were added or removed), then the sysroot image will use the cache and go straight on to the building step

This has the side effect of also making builds be truly incremental, but doesn't meet full criteria for #108 so I won't close that yet.

Fixes #106

[codecov[bot]]

Codecov Report

Merging #145 into master will increase coverage by 1.36%. The diff coverage is 90.47%.

@@            Coverage Diff             @@
##           master     #145      +/-   ##
==========================================
+ Coverage   87.11%   88.47%   +1.36%     
==========================================
  Files           6        7       +1     
  Lines         225      243      +18     
==========================================
+ Hits          196      215      +19     
+ Misses         29       28       -1

Flag	Coverage Δ
#unittests	88.47% <90.47%> (+1.36%)	:arrow_up:

Impacted Files	Coverage Δ
ros_cross_compile/dependencies.py	100% <100%> (ø)
ros_cross_compile/ros_cross_compile.py	73.46% <77.77%> (+6.02%)	:arrow_up:

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[emersonknapp]

Sorry everyone for the random review requests - i was trying to put the "approvers" team but it seems like it randomly picked individuals instead

[emersonknapp]

Also although this passes all current test cases, it does not allow for rosdep source customization as the README describes (via custom-data-dir and custom-setup-script).

On further thought, based on the amount of arbitrary customization users might want to do there, it may make sense to just expect users to have rosep installed and initialized locally. It's a Python package so we could import and call it, wouldn't even need subprocess - I'd appreciate any thoughts on that approach, I'll probably just implement it that way instead if nobody has an opinion. Regardless I'll close this PR for now until I have a regression test for customized rosdep sources.