Main packages without test files are ignored

[MichaelUrman]

In the latest run of coverpkg the following is reported:

$ coverpkg calc ./...
github.com/mutility/coverpkg/cmd/coverpkg-gha:       20.65%   76 of 368
github.com/mutility/coverpkg/internal/coverage:      66.29%  236 of 356
github.com/mutility/coverpkg/internal/git:            0.00%    0 of 22
github.com/mutility/coverpkg/internal/notes:          0.00%    0 of 49
<all>:                                               39.25%  312 of 795

However if a practically empty _test file is added to cmd/coverpkg, a new package shows up and the overall percentage drops:

$ coverpkg calc ./...
github.com/mutility/coverpkg/cmd/coverpkg:            0.00%    0 of 101
github.com/mutility/coverpkg/cmd/coverpkg-gha:       20.65%   76 of 368
github.com/mutility/coverpkg/internal/coverage:      66.29%  236 of 356
github.com/mutility/coverpkg/internal/git:            0.00%    0 of 22
github.com/mutility/coverpkg/internal/notes:          0.00%    0 of 49
<all>:                                               34.82%  312 of 896

As shown, internal/git and internal/notes show up with 0% regardless of test files, so this omission appears to be limited to main packages.