Refactor pytask to two packages, pytask and _pytask.

[tobiasraabe]

Changes

To separate the namespace of the public interface from the implementation, create a pytask and a _pytask package.

[codecov]

Codecov Report

Merging #14 into main will increase coverage by 11.30%.
The diff coverage is 94.20%.

@@             Coverage Diff             @@
##             main      #14       +/-   ##
===========================================
+ Coverage   83.63%   94.93%   +11.30%     
===========================================
  Files          37       26       -11     
  Lines        1949     1363      -586     
===========================================
- Hits         1630     1294      -336     
+ Misses        319       69      -250     

Flag	Coverage Δ
#end_to_end	84.29% <94.20%> (?)
#integration	52.89% <92.75%> (?)
#unit	90.16% <92.75%> (+6.53%)	⬆️

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Impacted Files	Coverage Δ
src/_pytask/dag.py	100.00% <ø> (ø)
src/_pytask/enums.py	100.00% <ø> (ø)
src/_pytask/exceptions.py	100.00% <ø> (ø)
src/_pytask/hookspecs.py	100.00% <ø> (ø)
src/_pytask/mark/expression.py	98.34% <ø> (ø)
src/_pytask/mark/structures.py	91.20% <ø> (ø)
src/_pytask/outcomes.py	100.00% <ø> (ø)
src/_pytask/report.py	100.00% <ø> (ø)
src/_pytask/session.py	100.00% <ø> (ø)
src/_pytask/shared.py	100.00% <ø> (ø)
... and 29 more

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