Add support for Cargo workspaces in Rust integration

[hobofan]

Without the PR Parcel tries to infer the output directory for a cargo build in a less flexible way. However, since the project structure of Cargo projects that make use of workspaces feature is slightly different from the normal one, that fails.

This PR uses the cargo metadata command to infer the correct target directory, which is should be fairly robust.

[codecov-io]

Codecov Report

Merging #1488 into master will increase coverage by 4.41%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1488      +/-   ##
==========================================
+ Coverage   86.41%   90.82%   +4.41%     
==========================================
  Files          80       80              
  Lines        4585     6248    +1663     
==========================================
+ Hits         3962     5675    +1713     
+ Misses        623      573      -50

Impacted Files	Coverage Δ
src/assets/RustAsset.js	91.95% <100%> (+0.28%)	⬆️
src/Logger.js	83.06% <0%> (-13.78%)	⬇️
src/packagers/CSSPackager.js	87.87% <0%> (-12.13%)	⬇️
src/utils/getRootDir.js	86.11% <0%> (-8.34%)	⬇️
src/worker.js	92.1% <0%> (-7.9%)	⬇️
src/HMRServer.js	85% <0%> (-6.67%)	⬇️
src/Bundle.js	93.05% <0%> (-6.2%)	⬇️
src/packagers/JSPackager.js	82.74% <0%> (-5.83%)	⬇️
src/Pipeline.js	93.06% <0%> (-4.89%)	⬇️
src/utils/bundleReport.js	67.04% <0%> (-4.75%)	⬇️
... and 43 more

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[devongovett]

Thanks!