Gazelle: extract import path from Go package comments and proto go_package options

[jayconrod]

Currently, Gazelle always sets the importpath attribute of a rule based on the prefix and its location within the repository. Gazelle should respect canonical import paths written in package comments.

package foo // import example.com/repo/foo

It should also use import paths from protos.

syntax = "proto3";

option go_package = "example.com/repo/foo";

[jayconrod]

Migrated to bazelbuild/bazel-gazelle#53