repo: implement :target suffix for package names

[kalikiana]

• Have you followed the guidelines for contributing?
• Have you signed the CLA?
• If this is a bugfix. Have you checked that there is a bug report open for the issue you are trying to fix on bug reports?
• If this is a new feature. Have you discussed the design on the forum?
• Have you successfully run ./runtests.sh static?
• Have you successfully run ./runtests.sh unit?

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Fixes: #1502
Related: LP: #1737561

[kyrofa]

(tests are failing)

[kalikiana]

These failures aren't mine: ERROR: test_build_maven_plugin (test_maven_plugin.MavenPluginTestCase) test_maven_plugin.MavenPluginTestCase.test_build_maven_plugin BlockingIOError: [Errno 11] write could not complete without blocking. I'll restart the job again.

[kalikiana]

Once more... apparently there's also a legitimate test_cross_compiling_with_cgo_and_target_suffix in there now.

[kalikiana]

Latest error output is this E: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/xenial-updates/main/binary-arm64/Packages 404 Not Found [IP: 91.189.88.152 80] so somehow the default sources are broken? I don't know how to test this... will close for now and discuss other options.

[lool]

(Is this the best place to discuss implementation?)

Just wanted to say that :target is nice and useful, but:

1. it would make sense to default to :target for stage-packages (and to build architecture for build-packages)
2. hence it might make sense to offer :build and or :host architectures as well