Add reproduction for broken package ignore

[Kingdutch]

The monorepo-builder configuration should ignore the package at packages/monorepo-builder/ignore/composer.json but the ExcludeFile iterator will only match against a sub-path.

Running composer merge demonstrates that the invalid package contained in the test composer.json ends up in our root composer.json which is undesirable.

Similarly using a relative path as argument to packageDirectoriesExcludes would not work either because the validation in that function requires the file to exist and it won't be able to properly resolve a relative file path.

Reproduces #4389