Execute prospector as a module (using -m)

[DonJayamanne]

Fixes #982

[codecov]

Codecov Report

Merging #1060 into master will increase coverage by 0.12%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #1060      +/-   ##
==========================================
+ Coverage   64.04%   64.17%   +0.12%     
==========================================
  Files         260      259       -1     
  Lines       12010    11838     -172     
  Branches     2133     2094      -39     
==========================================
- Hits         7692     7597      -95     
+ Misses       4309     4233      -76     
+ Partials        9        8       -1

Impacted Files	Coverage Δ
src/client/linters/linterInfo.ts	100% <ø> (ø)	⬆️
src/client/formatters/helper.ts	100% <ø> (ø)	⬆️
src/client/common/installer/productInstaller.ts	60.31% <50%> (-1.39%)	⬇️
src/client/common/featureDeprecationManager.ts	43.13% <0%> (-3.93%)	⬇️
src/client/linters/baseLinter.ts	87.5% <0%> (+2.08%)	⬆️
src/client/linters/linterManager.ts	85.5% <0%> (+2.89%)	⬆️
src/client/linters/pydocstyle.ts	95.12% <0%> (+4.87%)	⬆️
src/client/common/logger.ts	53.33% <0%> (+20%)	⬆️
src/client/linters/prospector.ts	78.94% <0%> (+63.15%)	⬆️

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[brettcannon]

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