[CT-1299] Make selected_resource non-global

[gshank]

Copied from #5088, which we are closing in favor of this because we're not actually seeing the test failure now.

Having a whole separate file for a global selected_resources variable in core/dbt/selected_resource.py is kind of overkill. It should be in some other file. In addition we should try hard to implement it in a way that doesn't use a global file variable, because they are a bad idea in general (despite the fact that we have a number of them) and they all need to be reset for tests to run properly (and quite possibly any persistent server code).

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[github-actions]

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