Modularize starfish.core.experiment.builder

[ttung]

We need to be able to import modules in starfish.core.experiment.builder without slurping in the whole thing. As it stands, if I add a dependency from starfish.core.imagestack to any module in starfish.core.experiment.builder, it creates a circular dependency. This moves the contents of starfish.core.experiment.builder.init.py to a standalone module.

The external api (starfish.experiment.builder) is maintained.

Test plan: make -j lint mypy fast-test

[codecov-io]

Codecov Report

Merging #1738 into master will increase coverage by <.01%.
The diff coverage is 95.79%.

@@            Coverage Diff             @@
##           master    #1738      +/-   ##
==========================================
+ Coverage   90.36%   90.36%   +<.01%     
==========================================
  Files         248      248              
  Lines        9167     9178      +11     
==========================================
+ Hits         8284     8294      +10     
- Misses        883      884       +1

Impacted Files	Coverage Δ
starfish/experiment/builder.py	0% <0%> (ø)	⬆️
...rfish/core/experiment/builder/test/test_inplace.py	98.61% <100%> (+0.01%)	⬆️
...sh/core/experiment/builder/structured_formatter.py	88.02% <100%> (ø)	⬆️
...ilder/test/structured_formatter/test_single_fov.py	100% <100%> (ø)	⬆️
...fish/core/imagestack/test/factories/all_purpose.py	100% <100%> (ø)	⬆️
...builder/test/structured_formatter/test_tilepath.py	100% <100%> (ø)	⬆️
starfish/core/imagestack/test/test_coordinates.py	96.47% <100%> (+0.04%)	⬆️
.../core/imagestack/test/factories/synthetic_stack.py	100% <100%> (ø)	⬆️
...ish/core/imagestack/test/test_slicedimage_dtype.py	95.06% <100%> (ø)	⬆️
...der/test/structured_formatter/test_multiple_fov.py	100% <100%> (ø)	⬆️
... and 7 more

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