feat: Handle absolute paths in XML config files (xml2json / readxml)

[kratsg]

Pull Request Description

• support XML configs with absolute paths by being able to prune the paths out/replace them to point elsewhere
• typehint the entire thing and fix some minor holes

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Add support for XML configurations with absolute paths by pruning the
paths out and renaming paths on-the-fly in readxml.
* Add -v/--mounts command line arguments for xml2json with similar behavior
to the Docker equivalent to support this feature.
* Add tests and test XML files with absolute paths.

[codecov]

Codecov Report

Merging #1909 (124ff59) into master (1ed7003) will decrease coverage by 0.03%.
The diff coverage is 95.45%.

@@            Coverage Diff             @@
##           master    #1909      +/-   ##
==========================================
- Coverage   98.22%   98.19%   -0.04%     
==========================================
  Files          68       68              
  Lines        4334     4365      +31     
  Branches      730      734       +4     
==========================================
+ Hits         4257     4286      +29     
- Misses         45       46       +1     
- Partials       32       33       +1     

Flag	Coverage Δ
contrib	26.25% <9.09%> (-0.10%)	⬇️
doctest	60.50% <52.27%> (-0.11%)	⬇️
unittests-3.10	96.08% <95.45%> (-0.02%)	⬇️
unittests-3.7	96.06% <95.34%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/pyhf/readxml.py	95.21% <93.33%> (-0.67%)	⬇️
src/pyhf/cli/rootio.py	92.59% <100.00%> (+0.28%)	⬆️
src/pyhf/utils.py	95.91% <100.00%> (+1.18%)	⬆️

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[matthewfeickert]

From @kratsg's testing so far it looks like this will be able to address Issue #1882. 👍

[kratsg]

@matthewfeickert this is ready, but codecov is broken for this one (some glitch on their side of things) probably related to my rebasing to clean things up.

[matthewfeickert]

Relevant RTD build: https://pyhf--1909.org.readthedocs.build/en/1909/_generated/pyhf.readxml.html

[matthewfeickert]

Thanks, this is looking super good.

I have a few questions and suggestions, but nothing major, just things that we could decide if we want to fixup now as they're getting added for the first time.

[matthewfeickert]

LGTM. 👍 Thanks @kratsg!