Hide PbList and PbMap constructors (#1072)

`PbList` and `PbMap` can't be constructed by the users properly, because the
constructor arguments are private:

- `PbList` element check function
- `PbMap` key and value field type arguments

Internally we had just one `PbMap` constructor call (with random field type
arguments) which we replaced with `{}`, and a few `PbList` constructor calls
(without a check function) which we replaced with `[]`.

To prevent confusion and incorrect uses of these types, hide the constructors.

Some of the `PbList` element validation tests are removed: they would need to
be moved to `protoc_plugin/test` as we no longer have access to `PbList`
constructors (but we can get `PbList`s from messages), but the tests are also
duplicates of the tests in `protoc_plugin/test/validate_fail_test.dart`. So
removed them instead.