Completions on single colons

[Veykril]

We seem to calculate completions for single colons of paths, we clearly shouldn't do that.

:$0 // shows completions
ResolvedThing:$0 // shows completions
UnresolvedThing:$0 // shows completions

[yue4u]

Not sure if I understand the issue correctly but trigger_character is None when the completion is triggered manually therefore following calculations happened. Related code: https://github.com/rust-lang/rust-analyzer/blob/master/crates/rust-analyzer/src/handlers.rs#L815-L825

[Veykril]

Ye looks like we just need to handle the None case there as well (and ideally add a test case for this, not sure if we have infra for testing trigger characters)