Updates to var, global macro resolution rules for package resources

[jtcohen6]

resolves #3572

What are you changing in this pull request and why?

• Update variable precedence rules, for resources in packages that are calling variables
• Copied a lot of Package-namespaced generate_x_name macro resolution dbt-core#7509 (review) into the docs on macro dispatch, and linked to that new subsection from generate_x_name pages

These are "quick hits," and very low-lying changes. For the people who have had to go deep on this stuff (package maintainers), I hope they're a big improvement to consistency/flexibility. I expect the vast majority of people to have no idea what these things mean.

Checklist

Uncomment if you're publishing docs for a prerelease version of dbt (delete if not applicable):

• Add versioning components, as described in Versioning Docs
• Add a note to the prerelease version Migration Guide
• Review the Content style guide and About versioning so my content adheres to these guidelines.
• Add a checklist item for anything that needs to happen before this PR is merged, such as "needs technical review" or "change base branch."

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[matthewshaver]

LGTM Thank you!