Source bash completion files for all other gz libraries

[azeey]

🎉 New feature

Toward #1
Replaces #103 (Creating a new PR with a branch on the upstream repo instead of a fork so I can test with a debbuilder job).

Summary

Source the gz*.completion file from wherever its installed so that the bash completion functions for other gz libraries can be picked up automatically.

Additionally, this installed the cmake configured etc/ign.bash_completion.sh file to <prefix>/share/bash-completion/completions/ign instead of relying on the debian/homebrew build infrastructure (see gazebo-release/gz-tools-release#6). This has two advantages (1) makes it easy to use the cmake configured file so we can have cmake variables in etc/ign.bash_completion.sh (2) tab completion works the same way when building from source if <prefix> is set to /usr.

Related:
gazebo-release/gz-tools-release#6

Test it

To build this with gazebo-release/gz-tools-release#6 on focal, symlink gz-tools-release/focal/debian into the the root directory of gz-tools, then from the root directory of gz-tools, build the deb files using dpkg-buildpackage -us -uc -b. The deb files will be created in the parent directory. You can then copy the debs to a docker container, install it, and test tab completion.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.