[SourceKit] Completion for declaration imports

[a7medev]

Motivation

Currently, SourceKit provides completion for importing modules or submodules as below.

// Import module
import |
// Import submodule
import Module.|

But it doesn't provide completion for importing specific declarations as follows.

import struct Module.|

Nor does it provide completion for declaration keywords like struct, class, and var after import.

Proposed solution

It should be possible to add completion for such cases in SourceKit.

Alternatives considered

No response

Additional information

No response