eagerly load CSV

[johnnychen94]

The "core" packages should still be loaded eagerly, otherwise, we'll hit a lot of world-age issues like #135.

closes #135

cc: @Dsantra92

[codecov-commenter]

Codecov Report

Merging #136 (c0dbf82) into master (5dad66d) will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #136      +/-   ##
==========================================
- Coverage   29.64%   29.55%   -0.10%     
==========================================
  Files          39       39              
  Lines        1565     1543      -22     
==========================================
- Hits          464      456       -8     
+ Misses       1101     1087      -14     

Impacted Files	Coverage Δ
src/MLDatasets.jl	100.00% <ø> (ø)
src/datasets/vision/mnist_reader/readlabels.jl	58.33% <0.00%> (-1.67%)	⬇️
src/datasets/vision/mnist_reader/readimages.jl	55.17% <0.00%> (-1.50%)	⬇️
src/datasets/text/udenglish.jl	56.00% <0.00%> (-0.87%)	⬇️
src/graph.jl	41.66% <0.00%> (-0.84%)	⬇️
src/datasets/graphs/planetoid.jl	100.00% <0.00%> (ø)
...rc/datasets/vision/cifar10_reader/CIFAR10Reader.jl	0.00% <0.00%> (ø)
.../datasets/vision/cifar100_reader/CIFAR100Reader.jl	0.00% <0.00%> (ø)
src/datasets/vision/cifar10.jl	2.43% <0.00%> (+0.02%)	⬆️
src/datasets/graphs/ogbdataset.jl	1.21% <0.00%> (+0.05%)	⬆️
... and 1 more

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[johnnychen94]

version resolving failure on nightly is unrelated oxinabox/DataDeps.jl#154

[CarloLucibello]

I guess this cannot be avoided unless we import whatever type can end up in CSV.File, which is probably InlineStrings, PooledArrays, SentinelArrays, WeakRefStrings based on the CSV.jl project file, but this doesn't seem much robust.
Another alternative is to @require CSV, but I don't want to have another breaking change, so let's go with this.