Ensure lerna plugins also work for lerna

[timkpaine]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 63.07692% with 24 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
lerna/core/plugins.py	45.45%	22 Missing and 2 partials ⚠️

Files with missing lines	Coverage Δ
lerna/tests/test_discover.py	100.00% <100.00%> (ø)
lerna/core/plugins.py	50.72% <45.45%> (-1.43%)	⬇️

... and 1 file with indirect coverage changes

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