Fix available_composite_ids including inline comp dependencies

[djhoese]

This PR fixes inline composite dependencies being listed in available_composite_ids/names and the all_composite_ids/names. The point of inline compositors is to not have to put as much work in to "officially" defining new composites and they shouldn't be needed by 99% of users. It can be confusing to see these dependencies (ex. airmass_dep_0) in the list coming from Scene.available_composite_ids.

This PR prefixes these inline dependencies with a _ and then ignores composites with that name when listing them in the Scene's methods.

• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff

[coveralls]

Coverage decreased (-0.02%) to 77.409% when pulling d3dc83f on djhoese:bugfix-inline-comp-dep-names into 2bfaf5a on pytroll:master.

[codecov]

Codecov Report

Merging #560 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #560      +/-   ##
=========================================
- Coverage   77.42%   77.4%   -0.02%     
=========================================
  Files         136     136              
  Lines       19139   19140       +1     
=========================================
- Hits        14819   14816       -3     
- Misses       4320    4324       +4

Impacted Files	Coverage Δ
satpy/composites/__init__.py	64.06% <100%> (ø)	⬆️
satpy/scene.py	84.06% <100%> (+0.03%)	⬆️
satpy/tests/compositor_tests/__init__.py	99.12% <100%> (ø)	⬆️
satpy/readers/__init__.py	92.98% <0%> (-1.41%)	⬇️

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[codecov]

Codecov Report

Merging #560 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #560      +/-   ##
=========================================
- Coverage   77.42%   77.4%   -0.02%     
=========================================
  Files         136     136              
  Lines       19139   19140       +1     
=========================================
- Hits        14819   14816       -3     
- Misses       4320    4324       +4

Impacted Files	Coverage Δ
satpy/composites/__init__.py	64.06% <100%> (ø)	⬆️
satpy/scene.py	84.06% <100%> (+0.03%)	⬆️
satpy/tests/compositor_tests/__init__.py	99.12% <100%> (ø)	⬆️
satpy/readers/__init__.py	92.98% <0%> (-1.41%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2bfaf5a...d3dc83f. Read the comment docs.

[djhoese]

The loss of test coverage is in a file I did not edit so not much I can do there. I'm going to merge this to rush a 0.11.1 release since the other @pytroll/core devs are on holiday.