x/tools/goimports: enable deleting without adding missing imports

Adding missing imports can take substantial time (~ 30-60 seconds in a large codebase) due to the need to load packages. This makes goimports problematic for "action on save" workflows, as saves would either block the editor or (if async) risk undoing user's work (when goimports eventually finishes and writes back the file).

I'd like to propose another flag, `-delete-only`, that'll produce only DeleteImport fixes, since I would assume that DeleteImport fixes can be produced quickly without loading packages either.

Start by locating the goimports flag handling and the code that produces DeleteImport and missing-import fixes. Trace how package loading is triggered, then verify that -delete-only emits only DeleteImport fixes without waiting for missing-import analysis.

Written by the indexing model from the issue text.