Merge #8794

8794: Give MergeBehaviour variants better names r=Veykril a=Veykril

I never really liked the variant names I gave this enum from the beginning and then I found out about rustfmt's `imports_granularity` config:

> imports_granularity
>
> How imports should be grouped into use statements. Imports will be merged or split to the configured level of granularity.
> 
>     Default value: Preserve
>     Possible values: Preserve, Crate, Module, Item
>     Stable: No

I personally prefer using `crate` over `full` and `module` over last, they seem more descriptive. Keeping these similar between tooling also seems like a good plus point to me.

We might even wanna take over the entire enum at some point if we have a `format/cleanup imports` assists in the future which would probably want to also have the `preserve` and `item` options.

Co-authored-by: Lukas Wirth <lukastw97@gmail.com>

crates/ide_completion/src/test_utils.rs

@@ -20,7 +20,7 @@ pub(crate) const TEST_CONFIG: CompletionConfig = CompletionConfig {
     add_call_argument_snippets: true,
     snippet_cap: SnippetCap::new(true),
     insert_use: InsertUseConfig {
-        merge: Some(MergeBehavior::Full),
+        merge: Some(MergeBehavior::Crate),
         prefix_kind: PrefixKind::Plain,
         group: true,
     },