Merge #2887

2887: Initial auto import action implementation r=matklad a=SomeoneToIgnore

Closes https://github.com/rust-analyzer/rust-analyzer/issues/2180

Adds an auto import action implementation.

This implementation is not ideal and has a few limitations:

* The import search functionality should be moved into a separate crate accessible from ra_assists.
This requires a lot of changes and a preliminary design. 
Currently the functionality is provided as a trait impl, more on that here: https://github.com/rust-analyzer/rust-analyzer/issues/2180#issuecomment-575690942

* Due to the design desicion from the previous item, no doctests are run for the new aciton (look for a new FIXME in the PR)

* For the same reason, I have to create the mock trait implementaion to test the assist

* Ideally, I think we should have this feature as a diagnostics (that detects an absense of an import) that has a corresponding quickfix action that gets evaluated on demand.
Curretly we perform the import search every time we resolve the import which looks suboptimal.
This requires `classify_name_ref` to be moved from ra_ide, so not done currently.

A few improvements to the imports mechanism to be considered later:

* Constants like `ra_syntax::SyntaxKind::NAME` are not imported, because they are not present in the database

* Method usages are not imported, they are found in the database, but `find_use_path` does not return any import paths for them

* Some import paths returned by the `find_use_path` method end up in `core::` or `alloc::` instead of `std:`, for example: `core::fmt::Debug` instead of `std::fmt::Debug`.
This is not an error techically, but still looks weird.

* No detection of cases where a trait should be imported in order to be able to call a method

* Improve `auto_import_text_edit` functionality: refactor it and move away from the place it is now, add better logic for merging the new import with already existing imports

Co-authored-by: Kirill Bulatov <[email protected]>

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