Mocking types referencing unsafe package or referencing non-existing types causes an error

[myshkin5]

Non-existing types are less of an issue because even if the mock is generated, won't compile. But x/tools/go/packages doesn't return any unsafe types (see golang/go#59929 and the code referencing the issue).

Expected results: users should be able to generate mocks referencing the unsafe package (and non-existing types sort of too).