Check if interpreter file exists before displaying in a list

[DonJayamanne]

Fixes #1305

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 #1352 into master will increase coverage by 0.02%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master    #1352      +/-   ##
==========================================
+ Coverage   71.16%   71.18%   +0.02%     
==========================================
  Files         269      269              
  Lines       12395    12401       +6     
  Branches     2198     2198              
==========================================
+ Hits         8821     8828       +7     
  Misses       3442     3442              
+ Partials      132      131       -1

Impacted Files	Coverage Δ
...nterpreter/locators/services/currentPathService.ts	95.23% <91.66%> (+0.79%)	⬆️
src/client/linters/baseLinter.ts	87.5% <0%> (-1.05%)	⬇️
...rc/client/debugger/PythonProcessCallbackHandler.ts	52.96% <0%> (+0.65%)	⬆️

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