Add NPE1Shim, to convert npe1 plugins to the npe2 interface

[tlambert03]

Still experimenting, but this PR would
a) discover npe1 plugins along with npe2 during the discovery process
b) have some sort of way to generate a manifest from the entrypoints (using same logic as npe2 convert)
c) cache that manifest for use on later launches

this could be used to help us transition to npe2 being the only plugin manager

[codecov]

Codecov Report

Merging #86 (fe34923) into main (41c557d) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              main       #86    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           23        24     +1     
  Lines         1355      1583   +228     
==========================================
+ Hits          1355      1583   +228     

Impacted Files	Coverage Δ
npe2/_command_registry.py	100.00% <100.00%> (ø)
npe2/_from_npe1.py	100.00% <100.00%> (ø)
npe2/_plugin_manager.py	100.00% <100.00%> (ø)
npe2/cli.py	100.00% <100.00%> (ø)
npe2/manifest/_npe1_shim.py	100.00% <100.00%> (ø)
npe2/manifest/schema.py	100.00% <100.00%> (ø)
npe2/manifest/utils.py	100.00% <100.00%> (ø)

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[nclack]

There was some good discussion in the plugin working group meeting. See notes.

Most of the discussion was around the caching behavior.

We discussed disable/uninstall support being tied to this which I think is great but probably a follow up pr?

[tlambert03]

OK @nclack, @Carreau, this is now ready to go. Due to the many moving parts, this has unfortunately ballooned into a massive PR. So I broke it up into three PRs.

1. NPE1Adapter Part 1 - updated _from_npe1 conversion logic to prepare for locally defined objects #124 - a more realistic test npe1 plugin, and logic to deal with locally defined objects in hookimpl scopes
2. NPE1Adapter Part 2 - adding the NPE1Adapter object. #125 - the shim object itself, discovery logic, and plugin_manager awareness
3. NPE1Adapter Part 3 - caching of adapter manifests #126 - caching logic and utilities

each PR contains the previous one... and #126 should actually be identical to this one. If you prefer, you can also simply review this one (and I would just close those PRs).

The whole thing can be tested in concert by checking out this branch (or the branch from #126) along with napari/napari#4015