Implement --build-for in addition to --platform

[cmatsuoka]

What needs to get done

When platforms: is used, craft applications should be able to select build platforms using --build-for, according to the following rules:

• If there's only one platform entry with the specified target architecture, choose that one.
• If that results in more than one entry, and there's one entry using the architecture name as the platform name, choose that one.
• Otherwise fail with an error.

Why it needs to get done

--build-for is extensively used and there's a reasonable way to map it to pick platform entries.

[syncronize-issues-to-jira[bot]]

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The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-2562.

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[sergiusens]

The second entry looks like it tries to solve for:

platforms:
  arm64:
  rpi4:
     build-on: arm64
     build-for: arm64