Make sure that any folder ending with site-packages is considered a library. Fixes #854

[fabioz]

Note that I haven't actually changed ptvsd not to issue CMD_SET_PROJECT_ROOTS as many tests have to be updated and I saw that there's a pull request open related to that.

I'm still a little unsure if this is actually the root cause for #854 as from my tests the location mentioned on it (/Users/vsts/Library/Python/3.7/lib/python/site-packages) already seemed to be treated as a library, so, it's possible this still doesn't fix the root cause (although I think it's a step in the right direction anyways and it should handle /usr/local/lib/python3.7/site-packages which wasn't really there and I could not find out a better heuristic to find about it).

[codecov-io]

Codecov Report

Merging #877 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #877   +/-   ##
=======================================
  Coverage   56.93%   56.93%           
=======================================
  Files          30       30           
  Lines        4177     4177           
=======================================
  Hits         2378     2378           
  Misses       1799     1799

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