Avoid matplotlib import overhead during 'import skimage'

[grlee77]

Description

This PR removes the overhead of importing matplotlib when doing import skimage (see figure in #5995 (comment)). I think the issue was unintentionally introduced by #5990. The solution used here is to move has_mpl out of skimage/_shared/__init__.py so it does not get imported before it is needed.

The following shows that with this PR there is no matplotlib import anymore:

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8
• Descriptive commit messages (see below)

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.