codecov counts vendored files

[ericphanson]

I vendored AbstractTrees and JuliaSyntax without their tests, so coverage is low. We aren't changing them so I don't think running the tests each commit would be useful anyway. But the low coverage annoys me, and https://github.com/JuliaTesting/ExplicitImports.jl/blob/main/codecov.yml did not fix it like I expected.

[giordano]

Maybe https://github.com/julia-actions/julia-processcoverage should have an option for filtering out directories, like it does for filtering? Or perhaps make it ignore directories starting with !?