clean up errors revealed by having a py.typed in poetry-core and running mypy in a proper environment

* more typechecking

* remove remaining `FooTypes` type annotations

just use the appropriate base class, no need for the extra complexity

* typecheck a couple more files

* more typechecking fixes

* more typechecking

* more typechecking

* tweak typing to appease pre-commit

* further mixology fixes

* consistency in repository package-finding

signal failure with `PackageNotFound`, rather than by returning None

* typechecking vcs.git.backend