Enable Snapcraft plugins in craft-application codepath

[tigarmo]

Two commits: one registers the plugins in the craft-application codepath, and one adds the confinement and base variables to the lifecycle (so that plugins can use it).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.33%. Comparing base (a019fe7) to head (dda0f79).
Report is 39 commits behind head on feature/craft-application.

Additional details and impacted files

@@                      Coverage Diff                      @@
##           feature/craft-application    #4672      +/-   ##
=============================================================
+ Coverage                      89.14%   89.33%   +0.19%     
=============================================================
  Files                            331      336       +5     
  Lines                          22078    22345     +267     
=============================================================
+ Hits                           19682    19963     +281     
+ Misses                          2396     2382      -14     

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[tigarmo]

This doesn't fix any spread tests but introduces a new, correct failure for tests/spread/core24/snap-creation:package_cutoff:

2024-03-15T18:57:19.2265172Z :: Python interpreter not found in payload.
2024-03-15T18:57:19.2266481Z :: + symlink_target=
2024-03-15T18:57:19.2267539Z :: + '[' -z '' ']'
2024-03-15T18:57:19.2268556Z :: + echo 'No suitable Python interpreter found, giving up.'
2024-03-15T18:57:19.2269497Z :: No suitable Python interpreter found, giving up.
2024-03-15T18:57:19.2270310Z :: + exit 1

[cmatsuoka]

We should add a test to make sure list-plugins is working as expected.

[tigarmo]

@sergiusens please rebase merge