colgon_plugin.py: rename colcon.py to colcon_plugin.py for consistency

[rpjday]

All other snapcraft plugin files have a standard naming convention.
Signed-off-by: Robert P. J. Day robert.day@canonical.com

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• Have you successfully run make lint?
• Have you successfully run pytest tests/unit?

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[codecov-commenter]

Codecov Report

Merging #4082 (26dce47) into main (0b95d4c) will not change coverage.
The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##             main    #4082   +/-   ##
=======================================
  Coverage   94.61%   94.61%           
=======================================
  Files         641      641           
  Lines       51903    51903           
=======================================
  Hits        49107    49107           
  Misses       2796     2796           

Impacted Files	Coverage Δ
snapcraft/parts/plugins/colcon_plugin.py	100.00% <ø> (ø)
snapcraft/parts/plugins/__init__.py	100.00% <100.00%> (ø)
snapcraft/parts/plugins/register.py	100.00% <100.00%> (ø)

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