Perform case insensitive file path comparisons on windows

[DonJayamanne]

Fixes #447
Note: This fix also requires a change in VSC Python Extension microsoft/vscode-python#1830

[codecov-io]

Codecov Report

Merging #457 into master will decrease coverage by 0.23%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #457      +/-   ##
==========================================
- Coverage   59.06%   58.82%   -0.24%     
==========================================
  Files          26       26              
  Lines        3498     3502       +4     
==========================================
- Hits         2066     2060       -6     
- Misses       1432     1442      +10

Impacted Files	Coverage Δ
ptvsd/wrapper.py	52.51% <0%> (-0.45%)	⬇️
ptvsd/_util.py	64.05% <0%> (-1.31%)	⬇️
ptvsd/ipcjson.py	68.03% <0%> (-0.46%)	⬇️

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