Add PluginManager dict() method to export state of manager

[tlambert03]

This adds a dict() method, akin to the BaseModel.dict() method on the manifests, that allows the state of the plugin manager to be exported, allowing for include and exclude arguments to be passed to each manifest dict() method.

This will come in handy for the npe2 list command line argument (#192 ) (but also provides a direct programmatic way)

In [8]: pm.dict(include={'contributions'}, exclude={'contributions.commands', 'contributions.themes'})
Out[8]:
{
    'plugins': {
        'napari-svg': {
            'contributions': {
                'readers': None,
                'writers': [
                    {'command': 'napari-svg.svg_writer', 'layer_types': ['image*', 'labels*', 'points*', 'shapes*', 'vectors*'], 'filename_extensions': ['.svg'], 'display_name': ''}
                ],
                'widgets': None,
                'sample_data': None,
                'menus': {},
                'submenus': None
            }
        }
    },
    'disabled': set(),
    'activated': set()
}

[codecov]

Codecov Report

Merging #197 (cd2f8e4) into main (d38a34c) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #197   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           30        30           
  Lines         2083      2106   +23     
=========================================
+ Hits          2083      2106   +23     

Impacted Files	Coverage Δ
npe2/_plugin_manager.py	100.00% <100.00%> (ø)
npe2/plugin_manager.py	100.00% <100.00%> (ø)

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[nclack]

lgtm