Fix RemoveUnusedImports command wiring

[amanthatdoescares]

Problem

The RemoveUnusedImports feature only considered imports at the top level of files, missing imports inside #if conditional compilation blocks. This led to:

• False negatives: Unused imports inside #if blocks weren't detected
• Incomplete cleanup: Projects with platform-specific or feature-flagged imports wouldn't get fully cleaned up

Solution

Implement recursive syntax tree traversal to find ALL imports throughout the entire file structure, including those inside conditional compilation blocks.
Fixes: #2335

Now properly handles all imports including those inside #if blocks while keeping platform-specific imports safe.

[ahoppen]

Thanks for opening the PR. It seems to me like it contains quite a few unrelated changes with possible regressions though. A couple things that jump out to me are (but there seem to be quite a few others). Could you ensure that your PR only focuses on the issue it aims to resolve?

• The removal of the comment on RemoveUnusedImportsCommand
• The removal of the filepath check for #if os(Windows)
• The removal of temporaryDocumentHasErrorDiagnostic

One of the particular challenges here is that we must not remove import statements in inactive #if clauses. For example, consider the following, which we have quite a few times in SourceKit-LSP. All of these import statements are necessary but on macOS, you can remove all but import Darwin without causing a compilation error. We should have a test for this that ensures only import statements from active #if clauses are removed?

#if canImport(Darwin)
import Darwin
#elseif canImport(Glibc)
import Glibc
#elseif canImport(Musl)
import Musl
#elseif canImport(CRT)
import CRT
#elseif canImport(Bionic)
import Bionic
#endif