Internal refactoring of CLI and test improvements

[browniebroke]

This change enables us to test the CLI without patching our own function, to have a test running the whole code path.

[sourcery-ai]

Sourcery Code Quality Report (beta)

❌  Merging this PR will decrease code quality in the affected files by 0.04 out of 10.

Quality metrics	Before	After	Change
Complexity	0.52	0.55	0.03 🔴
Method Length	34.56	36.01	1.45 🔴
Quality	9.11	9.07	-0.04 🔴

Other metrics	Before	After	Change
Lines	350	356	6

Changed files	Quality Before	Quality After	Quality Change
django_codemod/cli.py	8.76	8.66	-0.10 🔴
django_codemod/commands.py	9.50	9.52	0.02 🔵
tests/test_cli.py	9.20	9.12	-0.08 🔴
tests/visitors/base.py	9.66	9.78	0.12 🔵

Please see our documentation here for details on how these metrics are calculated.

We are actively working on this report - lots more documentation and extra metrics to come!

Let us know what you think of it via email or our Gitter!

[codecov]

Codecov Report

Merging #157 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #157   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           19        19           
  Lines          548       547    -1     
=========================================
- Hits           548       547    -1     

Impacted Files	Coverage Δ
django_codemod/cli.py	100.00% <100.00%> (ø)
django_codemod/commands.py	100.00% <100.00%> (ø)

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