Force lower-case during path normalization.

[FireSiku]

File paths are never case-sensitive. This fixes an issue that leads to false negatives when matching globs.

[mpeterv]

File paths are typically case-sensitive outside Windows.

[codecov-io]

Codecov Report

Merging #150 into master will decrease coverage by 0.2%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #150      +/-   ##
==========================================
- Coverage   97.77%   97.56%   -0.21%     
==========================================
  Files          35       35              
  Lines        4800     4801       +1     
==========================================
- Hits         4693     4684       -9     
- Misses        107      117      +10

Impacted Files	Coverage Δ
src/luacheck/fs.lua	88.17% <100%> (-3.14%)	:arrow_down:
src/luacheck/lua_fs.lua	84% <0%> (-12%)	:arrow_down:
src/luacheck/config.lua	91.86% <0%> (-1.63%)	:arrow_down:

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[FireSiku]

Oh. Right. Guess it never came up in my time using Ubuntu so brain just forgot about it. Fixed it.

[mpeterv]

Thank you!