NPE1Adapter Part 2 - adding the NPE1Adapter object.

[tlambert03]

Breaking up #86 into three steps. This is part 2 (but it contains #124 which can be reviewed first)

This adds the NPE1Adapter itself, and makes the plugin manager aware of it.

• in the discovery process, the _from_dist function now creates either a PluginManifest or an NPE1Shim based on the entry points offered by the plugin
• NPE1Adapter is a subclass of PluginManifest that can lazily load contributions (when manifest.contributions is accessed), by calling manifest_from_npe1 which was updated in NPE1Adapter Part 1 - updated _from_npe1 conversion logic to prepare for locally defined objects #124
• This lazily loaded manifest is used for the duration of the session, (caching behavior will be added in step 3)
• The plugin manager itself now saves a list of _npe1_adapters during plugin discovery and gains a new method index_npe1_adapters that napari can use to force importing (or loading from cache) of all npe1 plugins.

Caching behavior comes in the next PR

[codecov]

Codecov Report

Merging #125 (bc9c5b9) into main (cb45d7d) will increase coverage by 1.89%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##             main      #125      +/-   ##
===========================================
+ Coverage   98.10%   100.00%   +1.89%     
===========================================
  Files          24        25       +1     
  Lines        1581      1650      +69     
===========================================
+ Hits         1551      1650      +99     
+ Misses         30         0      -30     

Impacted Files	Coverage Δ
npe2/_from_npe1.py	100.00% <100.00%> (+4.21%)	⬆️
npe2/_plugin_manager.py	100.00% <100.00%> (ø)
npe2/manifest/_npe1_adapter.py	100.00% <100.00%> (ø)
npe2/manifest/schema.py	100.00% <100.00%> (ø)
npe2/manifest/utils.py	100.00% <0.00%> (+12.59%)	⬆️

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[nclack]

looks good. The docs in _from_npe1 are good. I thought a comment giving a bit of a high level introduction to what the shim was could go in _npe1_shim.py.

[tlambert03]

I thought a comment giving a bit of a high level introduction to what the shim was could go in _npe1_shim.py.

renamed to NPE1Adapter and added a longer docstring to NPE1Adapter explaining a bit more how it works.