Implement conditional dependencies and subdependency mapping/passthrough

[qnighy]

Why

This is a PR for the long-awaited features: conditional dependencies and subdependency mapping/passthrough. In short, computed fields can now accept subdeps.

conditional dependencies

Load foo only when use_foo is specified.

dependency foo: -> (subdeps) { subdeps.normalized[:use_foo].any? }
computed def bar; end

subdependency mapping

Load foo with subdeps specified in foospec.

dependency foo: -> (subdeps) { subdeps.normalized[:foospec] }
computed def bar; end

subdependency passthrough

Delegate subdeps as-is.

dependency foo: -> (subdeps) { subdeps }
computed def bar; end

What

To support conditional dependencies, the following changes are made to the interpretation of subdeps:

• A callable object will be evaluated if it's in a field's dependency list. If it returns an array, it's flattened.
• subdeps must have at least one truthy value. Otherwise the dependency is considered disabled.

To maintain compatibility and intuition, the normalization process is also changed:

• :foo or [:foo] will be normalized into { foo: [true] } (previously { foo: [] }).
• { foo: [] } will be normalized into { foo: [true] } (previously { foo: [] }).

The values nil, false, true are filtered out in the following cases:

• Before passed to a loader (defined by define_loader or define_primary_loader).
• Before normalization in -> (subdeps) { subdeps.normalized ... }. Note that the subdeps parameter itself keeps these values as-is.

[codecov]

Codecov Report

Merging #25 (445026d) into master (cb7ff0c) will increase coverage by 0.62%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
+ Coverage   94.29%   94.92%   +0.62%     
==========================================
  Files           4        4              
  Lines         228      256      +28     
  Branches       41       48       +7     
==========================================
+ Hits          215      243      +28     
  Misses          7        7              
  Partials        6        6              

Impacted Files	Coverage Δ
lib/computed_model/plan.rb	100.00% <ø> (ø)
lib/computed_model.rb	100.00% <100.00%> (ø)
lib/computed_model/dep_graph.rb	100.00% <100.00%> (ø)
lib/computed_model/model.rb	88.49% <100.00%> (+0.53%)	⬆️

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