cmd/govim: provide commands to add/remove imports (with completion)

[myitcv]

Whether provided by gopls or not, govim should provide the ability to add/remove an import. Additionally, it should be possible to tab complete the import to add. Fuzzy matching could/should be provided by an external plugin, something like fzf.

e.g.

:GOVIMAddImport en<Tab>

... should then provide all the imports that start with (or contain, I think Vim defines these semantics somewhere) en. Which could include encoding/json.

If this is not provided by gopls, then govim should watch the main module and pre-cache the list of packages that can be imported.

[myitcv]

Discussion continuing in golang/go#32749. Need some movement there first