PyB plugins use wrong external tool dependencies

[arcivanov]

Throughout the PyB internal plugins use project.build_depends_on:

• Flake8
• Coverage

Firstly, PyB build does not really depend on those. It's the runtime use of the plugin that requires these external tools.

Therefore, I propose the following:

• Create another method project.plugin_depends_on(package_name, version_spec=None, external_tool=None)
  • package_name is package name 😄
  • version_spec, if specified is a required PEP0440 SpecifierSet (see Plugin-specified PyB API version constraints #271)
  • external_tool if specified is then name of the executable to check with assert_can_execute

project.plugin_depends_on will:

• via pip show <package_name> check the version of the package installed
• if nothing is installed, pip install <package_name>[version_spec] will be executed
• if package is installed but version_spec is specified and version is not contained in SpecifierSet pip install --upgrade <package_name>[version_spec] will be run
• once package versions are settled, if external_tool is specified the assert_can_execute will be performed.

@mriehl @esc review please

[arcivanov]

@mriehl @esc thoughts? I'm going to start implementing as stated, please weigh in if you have objections