Fix: `cruft diff` fails for `cookiecutter` local extensions

[fabrykato]

This PR aims to close #289 and finishes support for local extensions as started in #237.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.72%. Comparing base (1a5614f) to head (d829bcc).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #290   +/-   ##
=======================================
  Coverage   99.71%   99.72%           
=======================================
  Files          21       21           
  Lines        1050     1086   +36     
=======================================
+ Hits         1047     1083   +36     
  Misses          3        3           

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[fabrykato]

I'd like to fix the coverage problem (1 more line missing than before), but I'd have to add a cookiecutter-test branch not based on sub-directory cookiecutter template for that.

[fabrykato]

I'd like to fix the coverage problem (1 more line missing than before), but I'd have to add a cookiecutter-test branch not based on sub-directory cookiecutter template for that.

Based on https://github.com/Fabi89/cookiecutter-test/tree/no-dir-extensions and https://github.com/Fabi89/cookiecutter-test/tree/no-dir-extensions-update