Refactor metadata initialisation, model search, and external loading of model code

[ablaom]

• Refactor the metadata initialization code and model search functionality to eliminate speed issues and improve maintainability

• Improve test coverage and fix some undiscovered bugs in loading model code with @load

• Add macro-free method for loading model code into an arbitrary module. Do ?load_implementation for details.

• Tweak scitype(::Model) definition to make the return value a Julia type, rather than an instance.

[codecov-io]

Codecov Report

Merging #215 into master will decrease coverage by <.01%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master     #215      +/-   ##
==========================================
- Coverage   69.39%   69.39%   -0.01%     
==========================================
  Files          20       23       +3     
  Lines        1552     1614      +62     
==========================================
+ Hits         1077     1120      +43     
- Misses        475      494      +19

Impacted Files	Coverage Δ
src/tuning.jl	71.42% <ø> (ø)	⬆️
src/MLJ.jl	100% <100%> (+28.57%)	⬆️
src/loading.jl	100% <100%> (+21.91%)	⬆️
src/scitypes.jl	14.28% <14.28%> (ø)
src/model_search.jl	51.61% <51.61%> (ø)
src/metadata.jl	92.85% <92.85%> (ø)
src/builtins/Transformers.jl	78.88% <0%> (-0.56%)	⬇️

---

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 d3dca37...36fdec6. Read the comment docs.