Fix --config and --rootdir inconsistencies

[mondeja]

Fixes #171

[codecov]

Codecov Report

Base: 97.02% // Head: 96.90% // Decreases project coverage by -0.12% ⚠️

Coverage data is based on head (51b49b2) compared to base (de6b2a8).
Patch coverage: 94.11% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #176      +/-   ##
==========================================
- Coverage   97.02%   96.90%   -0.13%     
==========================================
  Files          43       43              
  Lines        2826     2840      +14     
==========================================
+ Hits         2742     2752      +10     
- Misses         84       88       +4     

Impacted Files	Coverage Δ
src/project_config/__main__.py	98.87% <75.00%> (-1.13%)	⬇️
src/project_config/commands/check.py	99.18% <100.00%> (+0.01%)	⬆️
src/project_config/config/__init__.py	95.34% <100.00%> (-1.27%)	⬇️
src/project_config/config/exceptions.py	100.00% <100.00%> (ø)

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