[CT-2470] Package-namespaced macro resolution

[MichelleArk]

Resources in a package should be rendered with a rendering context specific to that package (its vars and macros). This issue focuses on macros resolution.

Acceptance Criteria

• Macros defined in imported packages should be respected when compiling code defined in same imported package (this is already the case - just not for generate_x_macros)

• MacroManifest.find_generate_macro_by_name should consider imported packages if the calling node is defined in an imported package:

  dbt-core/core/dbt/contracts/graph/manifest.py

  Lines 575 to 576 in fd73066

  	The `generate_X_name` macros are similar to regular ones, but ignore
  	imported packages.

  • e.g. package_a uses its version of get_custom_alias, package_b uses its version of get_custom_alias during parse-time

• If the root project wants to override a package_a macro being called within a package_a model, it would be possible to achieve this desirable behavior with dispatch — as long as the package_a macro in question is dispatched to the package_a namespace.