Context: https://github.com/dotnet/maui/pull/3018#pullrequestreview-792369556
In order for the .NET MAUI workload to properly implement implicit
global usings:
1. The .NET MAUI workload will add many `@(Using)` entries that
conflict with each platform's APIs.
2. We need *something* to identify `@(Using)` is for a specific
platform, so we can use a new `%(Platform)` metadata for this.
3. Late in .NET MAUI's MSBuild targets, we can do:
<ItemGroup Condition=" '$(UseMaui)' == 'true' and ('$(ImplicitUsings)' == 'true' or '$(ImplicitUsings)' == 'enable') ">
<Using Remove="@(Using->HasMetadata('Platform'))" />
</ItemGroup>
In .NET 7, we might have a nicer design around this, but for now this
is the plan for .NET 6.
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