Allow removal of implicit `gradlePluginPortal()` repository

[lacasseio]

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We should also consider adding an override for the default gradlePluginPortal() repository. In scenarios where no repositories are defined, a default, implicit, gradlePluginPortal() repositories is declared. However, as soon as one repository is used, no default repository is declared. We account for this scenario so our Nokee repository feels like it is appended.

Something, like: dev.nokee.repository.gradlePluginPortal=(implicit|enabled|disabled)