Associate default prefixes with external repositories

[jayconrod]

(Migrated from bazel-contrib/rules_go#606)

We should associate prefixes with external repository names. These prefixes would be used transform import paths to Bazel labels for repositories that haven't been fetched and can't be indexed yet (see #12).

In the examples below, we would transform dependencies on github.com/example/project to @repo_with_custom_name//:go_default_library.

For go_repository, we can infer this from importpath or remote.

go_repository(
    name = "repo_with_custom_name",
    importpath = "github.com/example/project",
)

We may be able to do this for git_repository, too, in some cases.

git_repository(
    name = "repo_with_custom_name",
    remote = "https://github.com/example/project",
)

This wouldn't work with import path redirection. In particular, it wouldn't work with golang.org, gopkg.in, any fork, or private repositories. Gazelle should recognize directives in WORKSPACE to override the import path.

# gazelle:importpath github.com/example/project
git_repository(
    name = "repo_with_custom_name",
    remote = "https://github.com/fork/project",
)

If Gazelle can't discover the repository by parsing WORKSPACE (i.e., if a repository is declared in a macro), we should be able to read standalone declarations that map the prefix to the repo name.

# gazelle:importpath repo_with_custom_name github.com/example/project