[Tidy first] Task config type

[gshank]

Tidy first changes to add a type to "config" in the task hierarchy.

This involved moving a couple of things around because it had a badly constructed inheritance hierarchy, in which the base class did things that only applied to certain subclasses.

And of course, as usual when fixing mypy typing, it then started complaining about a couple of things which weren't all that related. Note that PackageContract and PackageSpec were the same Union, so I consolidated on PackageSpec.

[codecov]

Codecov Report

Attention: Patch coverage is 82.00000% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 88.10%. Comparing base (b945d17) to head (e642580).
Report is 2 commits behind head on main.

Files	Patch %	Lines
core/dbt/task/base.py	50.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9874      +/-   ##
==========================================
+ Coverage   88.06%   88.10%   +0.04%     
==========================================
  Files         178      178              
  Lines       22457    22454       -3     
==========================================
+ Hits        19776    19784       +8     
+ Misses       2681     2670      -11     

Flag	Coverage Δ
integration	85.47% <76.00%> (+0.05%)	⬆️
unit	61.87% <50.00%> (-0.04%)	⬇️

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