Use an existing method to identify the active interpreter

[DonJayamanne]

Fixes #1015

This pull request:

• Has a title summarizes what is changing
• Includes a news entry file (remember to thank yourself!)
• Has unit tests & code coverage is not adversely affected (within reason)
• Works on all actively maintained versions of Python (e.g. Python 2.7 & the latest Python 3 release)
• Works on Windows 10, macOS, and Linux (e.g. considered file system case-sensitivity)

[codecov]

Codecov Report

Merging #1215 into master will decrease coverage by 0.18%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1215      +/-   ##
==========================================
- Coverage   72.21%   72.03%   -0.19%     
==========================================
  Files         260      260              
  Lines       11933    11928       -5     
  Branches     2121     2119       -2     
==========================================
- Hits         8618     8592      -26     
- Misses       3188     3214      +26     
+ Partials      127      122       -5

Impacted Files	Coverage Δ
src/client/interpreter/contracts.ts	100% <ø> (ø)	⬆️
src/client/common/installer/moduleInstaller.ts	48.14% <100%> (-2.7%)	⬇️
src/client/linters/errorHandlers/notInstalled.ts	33.33% <0%> (-61.12%)	⬇️
src/client/linters/errorHandlers/errorHandler.ts	77.77% <0%> (-22.23%)	⬇️
src/client/common/logger.ts	53.33% <0%> (-20.01%)	⬇️
src/client/common/application/applicationShell.ts	23.07% <0%> (-7.7%)	⬇️
src/client/common/installer/productInstaller.ts	60.31% <0%> (-4.24%)	⬇️
src/client/linters/baseLinter.ts	87.5% <0%> (-4.17%)	⬇️
...rc/client/debugger/PythonProcessCallbackHandler.ts	53.61% <0%> (+1.31%)	⬆️
src/client/debugger/PythonProcess.ts	48.75% <0%> (+1.66%)	⬆️

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