Support CLCACHE_BASEDIR in nodirect mode

[oktal3700]

Do a simple case-insensitive find-and-replace to transform absolute paths into relative paths within the preprocessor output that is used to compute the hash. This makes CLCACHE_NODIRECT mode usable in the presence of the __FILE__ macro.

Fixes a bug in _normalizedCommandLine that caused the source filename to be included in the hash computation.

[oktal3700]

Not sure what caused the build to fail.

[oktal3700]

it also invalidates all existing cache entries for people who do not use the direct mode

Are you sure? Only computeKeyNodirect is affected. The bugfix part will invalidate cache entries for people who do not use BASEDIR though, is that what you meant?

[codecov-io]

Codecov Report

Merging #349 into master will increase coverage by 0.03%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #349      +/-   ##
==========================================
+ Coverage   88.93%   88.96%   +0.03%     
==========================================
  Files           4        4              
  Lines        1301     1305       +4     
  Branches      195      196       +1     
==========================================
+ Hits         1157     1161       +4     
  Misses        106      106              
  Partials       38       38

Flag	Coverage Δ
#integrationtests_memcached	67.71% <100%> (+0.1%)	:arrow_up:
#unittests	85.9% <100%> (+0.04%)	:arrow_up:

Impacted Files	Coverage Δ
clcache/__main__.py	90.9% <100%> (+0.03%)	:arrow_up:

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[oktal3700]

Applied suggested changes.