Variables: Resign from projectDir concept

[medikoo]

As decided internally, resign from projectDir concept.

After that change, files in variables can be resolved from any system path

[codecov]

Codecov Report

Merging #10478 (8f84e58) into master (129c0ee) will decrease coverage by 0.00%.
The diff coverage is n/a.

❗ Current head 8f84e58 differs from pull request most recent head 105c76a. Consider uploading reports for the commit 105c76a to get more accurate results

@@            Coverage Diff             @@
##           master   #10478      +/-   ##
==========================================
- Coverage   85.89%   85.88%   -0.01%     
==========================================
  Files         335      335              
  Lines       13934    13931       -3     
==========================================
- Hits        11968    11965       -3     
  Misses       1966     1966              

Impacted Files	Coverage Δ
lib/configuration/variables/sources/file.js	100.00% <ø> (ø)

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[pgrzesik]

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