Add Debian 'Replaces: colcon' field

[cottsay]

Upstream Debian has decided to package colcon, and they created a package simply called 'colcon' which provides only the /usr/bin/colcon executable and some weak dependencies as a sort of replacement for colcon-common-extensions. This conflicts with our python3-colcon-core package during unpacking.

We could add Conflicts: colcon, but I think this is a great use case for Replaces: because the only file provided by colcon is /usr/bin/colcon, which we obviously provide here. This way, folks can still successfully install colcon and get the weak dependencies even if our version of python3-colcon-core is installed, and our copy of /usr/bin/colcon will take precedence over the one provided by colcon.

https://www.debian.org/doc/debian-policy/ch-relationships.html#overwriting-files-and-replacing-packages-replaces

[codecov[bot]]

Codecov Report

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

Project coverage is 83.54%. Comparing base (03d6193) to head (a0ee3d9).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #628 +/- ## ======================================= Coverage 83.54% 83.54% ======================================= Files 66 66 Lines 3816 3816 Branches 745 745 ======================================= Hits 3188 3188 Misses 554 554 Partials 74 74 ```

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