Hide PbList and PbMap constructors

[osa1]

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 PbLists from messages), but the tests are also
duplicates of the tests in protoc_plugin/test/validate_fail_test.dart. So
removed them instead.

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:protobuf	6.0.0	ready to publish	protobuf-v6.0.0
package:protoc_plugin	24.0.0	ready to publish	protoc_plugin-v24.0.0

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[osa1]

Removed because we no longer have access to PbList. We could test these using generated messages (by moving the test to protoc_plugin/test), but we already test the same things in https://github.com/google/protobuf.dart/blob/master/protoc_plugin/test/validate_fail_test.dart so these are also redundant.

[osa1]

(Waiting to test this internally after cl/825440440)