Issue description: The language team decided that it is a compile-time
error for a dot shorthand to access the static namespace of an
inaccessible private declaration. This CL updates code completion to
avoid suggesting inaccessible private declarations.

What is the fix: Private declarations from another library aren't
suggested by code completion.

Why cherry-pick: Avoid encouraging users to use this behaviour when it's
not fixed yet. Once the fix lands, this will prevent code completion
from suggesting erroneous code.

Risk: Low, this fix only affects code completion.

Issue link: #62416

Cherry-pick: https://dart-review.googlesource.com/c/sdk/+/473300
Change-Id: I55201e76283ee333b48488871b6254de102466f1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/474490
Reviewed-by: Leaf Petersen <leafp@google.com>
Commit-Queue: Kallen Tu <kallentu@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>