refactor: use absolute imports

[alexander-held]

Motivated by scikit-hep/pyhf#1539, this switches from relative to absolute imports within cabinetry. Reasons are outlined in scikit-hep/pyhf#1539 (comment), reproduced below:

The imports section of PEP 8 says

Absolute imports are recommended, as they are usually more readable and tend to be better behaved (or at least give better error messages) if the import system is incorrectly configured

and

However, explicit relative imports are an acceptable alternative to absolute imports, especially when dealing with complex package layouts where using absolute imports would be unnecessarily verbose

PEP 8 used to discourage relative imports more strongly.

The Google style guide section says

Do not use relative names in imports. Even if the module is in the same package, use the full package name. This helps prevent unintentionally importing a package twice.

The relative imports used previously were originally motivated by pyhf, so it seems appropriate to switch to absolute imports together with pyhf.

* switch from relative to absolute imports

[codecov]

Codecov Report

Merging #252 (f58ce92) into master (1043103) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #252   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           17        17           
  Lines         1720      1720           
  Branches       261       261           
=========================================
  Hits          1720      1720           

Impacted Files	Coverage Δ
src/cabinetry/__init__.py	100.00% <100.00%> (ø)
src/cabinetry/cli/__init__.py	100.00% <100.00%> (ø)
src/cabinetry/fit.py	100.00% <100.00%> (ø)
src/cabinetry/route.py	100.00% <100.00%> (ø)
src/cabinetry/template_builder.py	100.00% <100.00%> (ø)
src/cabinetry/template_postprocessor.py	100.00% <100.00%> (ø)
src/cabinetry/visualize/__init__.py	100.00% <100.00%> (ø)
src/cabinetry/visualize/plot_model.py	100.00% <100.00%> (ø)
src/cabinetry/visualize/plot_result.py	100.00% <100.00%> (ø)
src/cabinetry/workspace.py	100.00% <100.00%> (ø)

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