Refactor of @load

[ablaom]

Addresses JuliaAI/MLJ.jl#693 .

• The intended behaviour of @load is not changed by this PR with this exception: By default verbosity=1, so that newcomers understand what the macros is doing. I think this has been a source of confusion a number of times.

julia> @load AdaBoostStumpClassifier
[ Info: For silent loading, specify `verbosity=0`. 
import MLJDecisionTreeInterface ✔
const AdaBoostStumpClassifier = MLJDecisionTreeInterface.AdaBoostStumpClassifier ✔
AdaBoostStumpClassifier(
    n_iter = 10,
    pdf_smoothing = 0.0) @036

• to load code without returning an instance of the new model type, there is a new macro @loadcode

• Support for the load function is removed. Instead, for explicit importing of model types without a macro, the user may extract the appropriate load path using the load_path function, a model trait now overloaded to work on strings and model registry entries:

julia> load_path("PCA")
"MLJMultivariateStatsInterface.PCA"

julia> load_path("LinearRegressor", pkg="GLM")
"MLJModels.GLM_.LinearRegressor"

julia> load_path(models()[1])
"MLJScikitLearnInterface.ARDRegressor"

TODO:

• Test @load works from within modules and packages (provided interface is not Requires-loaded - a known issue, see lazy activation of models not working from within packages #22)

[ablaom]

@DilumAluthge @ExpandingMan

[ExpandingMan]

I was thinking maybe instead of @loadcode it can be called @import? It seems appropriate since the behavior is closely aligned with the functionality of the actual Julia keyword import, but perhaps using up the namespace that much is a bit aggressive.

[DilumAluthge]

@import seems fine to me.

[ablaom]

@import it is then It would seem this is not allowed??

julia> macro import() quote end end
ERROR: syntax: invalid name "import"
Stacktrace:
 [1] top-level scope at none:1

[OkonSamuel]

@import it is then It would seem this is not allowed??

julia> macro import() quote end end
ERROR: syntax: invalid name "import"
Stacktrace:
 [1] top-level scope at none:1

@ablaom @loadcode sounds a little off, what of renaming to @loadmodel or @ml_import

[ExpandingMan]

Hm, yeah it might protect the keyword import. It was probably a bad idea to shadow it in the first place.