support @lazyload for lazy loading of script dependencies

[Roger-luo]

supersedes #204

[codecov]

Codecov Report

Merging #205 (b4634ab) into master (9f12779) will decrease coverage by 16.29%.
The diff coverage is 73.33%.

@@             Coverage Diff             @@
##           master     #205       +/-   ##
===========================================
- Coverage   83.24%   66.94%   -16.30%     
===========================================
  Files          21       21               
  Lines        1468     1570      +102     
===========================================
- Hits         1222     1051      -171     
- Misses        246      519      +273     

Impacted Files	Coverage Δ
src/frontend/cast.jl	77.77% <73.33%> (-3.66%)	⬇️
src/builder/app.jl	0.00% <0.00%> (-100.00%)	⬇️
src/builder/sysimg.jl	0.00% <0.00%> (-84.62%)	⬇️
src/builder/install.jl	33.78% <0.00%> (-59.46%)	⬇️
src/builder/tarball.jl	0.00% <0.00%> (-57.15%)	⬇️
src/ast/printing.jl	60.66% <0.00%> (-29.28%)	⬇️
src/builder/cli.jl	66.17% <0.00%> (-23.53%)	⬇️
src/frontend/utils.jl	69.56% <0.00%> (-17.40%)	⬇️
src/configs.jl	62.33% <0.00%> (-15.59%)	⬇️
src/ast/types.jl	94.28% <0.00%> (-5.72%)	⬇️
... and 5 more

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