Ensure System.Resources.Extensions targets are applied transitively

[ericstj]

Copy targets to buildTransitive so that they will be applied from
transitive package/project references. We maintain build folder so that
old nuget clients can still get the build targets.

Fixes #50286

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Issue Details

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Copy targets to buildTransitive so that they will be applied from
transitive package/project references. We maintain build folder so that
old nuget clients can still get the build targets.

Fixes #50286

Author:	ericstj
Assignees:	ericstj
Labels:	area-System.Resources
Milestone:	-

[ericstj]

For reference here is the buildTransitive spec: https://github.com/NuGet/Home/wiki/Allow-package--authors-to-define-build-assets-transitive-behavior#solution

Duplicating targets is the "best practice" here for back-compat.

[ericstj]

Thanks @jnm2 for the great report of this issue.

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[ericstj]

Opened #50300 to track the mono failure. Looks interesting 🤨 not related to this change though.