Locate .coverage_contracts correctly for subfolder paths

[cgewecke]

#559

Fixes a bug where the temporary contracts directory is always placed in the root of the project.

When contracts_directory or paths.sources references a subfolder, e.g contracts/legacy and imports from adjacent folders, none of the Solidity import statements resolve correctly.

In this case, .coverage_contracts should be a subfolder of contracts.

[codecov-io]

Codecov Report

Merging #570 (423a9c2) into master (f5ed5e5) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #570   +/-   ##
=======================================
  Coverage   99.05%   99.05%           
=======================================
  Files          22       22           
  Lines         955      955           
=======================================
  Hits          946      946           
  Misses          9        9           

Impacted Files	Coverage Δ
plugins/resources/plugin.utils.js	97.61% <100.00%> (ø)

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