x/tools/cmd/guru: change package referrers to show imports rather than qualified identifiers

[josharian]

cmd/guru's packageReferrers contains this TODO:

			// TODO(adonovan): perhaps more useful would be to show imports
			// of the package instead of qualified identifiers.

This seems more useful, because there'd be fewer results, which means the user is less likely to drown in them. It also admits of a dramatically cheaper implementation, because it could be done entirely via go/parser, with parser.ImportsOnly.

@alandonovan how do you feel about pulling the trigger on this?

cc @keegancsmith -- see https://github.com/sourcegraph/go-langserver/blob/feba79e88e9cca61e450f5c15d2c01d597e2d46c/langserver/references.go#L59

[spf13]

ping @alandonovan for decision

[adonovan]

guru was deleted in #65880; closing as obsolete. Incidentally, gopls gets this right.