Move the package package override checks out of colcon-core

[cottsay]

This functionality is being moved to a separate colcon extension, colcon-override-check.

Reverts part of #449

[codecov]

Codecov Report

Merging #476 (85e4671) into master (64f06cf) will increase coverage by 0.66%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #476      +/-   ##
==========================================
+ Coverage   81.11%   81.77%   +0.66%     
==========================================
  Files          59       59              
  Lines        3569     3540      -29     
  Branches      687      676      -11     
==========================================
  Hits         2895     2895              
+ Misses        628      599      -29     
  Partials       46       46              

Impacted Files	Coverage Δ
colcon_core/verb/build.py	0.00% <ø> (ø)

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[sloretz]

LGTM when the replacement is merged

[cottsay]

I'm holding this PR until colcon/colcon-common-extensions#15 is merged, which itself is waiting until I have sufficient privileges to release that package.
Merging this prematurely could mean that either we're without override checks or blocking colcon-core releases until colcon-common-extensions can be released.