Drop support for .packages files

[jakemac53]

This package should drop support for .packages files, and set a minimum SDK of 3.0.0 or higher (nvm, the min SDK is already sufficiently high).

Reasoning:

• As of 3.x, only versions >= 2.12 are supported.
• The .packages file behavior sets a version of 2.7, so it is not useful on SDKs >=3.0.0
• If a .packages file is discovered, and there isn't a corresponding .dart_tool/package_config.json file, it will select that, instead of traversing further up the chain.
• Specifically when refactoring to use pub workspaces, you can end up in a state where you have old .packages files but no .dart_tool/package_config.json file. This makes for example the formatter go off the rails and parse all files with language version 27.
• It should also make the logic a bit simpler and faster to stop looking for these files.

[jakemac53]

Actually I see now that a minVersion is supported - but it defaults to 1. I think we should at least change the default to 2. I don't believe that supporting version one provides any positive value, and it can provide negative value. So, I would opt for dropping support entirely, but leaving it in as an opt in mechanism could also be OK (although, I don't see any useful scenario for using version 1, it cannot produce a valid package config in Dart 3.x).

[lrhn]

Ack. We should just kill support for .packages and have an SDK constraint of ^3.0.0.