Directive to declare repository rules defined in macros

[jayconrod]

Gazelle should understand directives in WORKSPACE that describe repositories declared using a mechanism other than go_repository (for example, a local_repository or an http_archive or something declared inside a macro). The directive should tell Gazelle the name of the repository and it's prefix.

# gazelle:repository <repo_name> <prefix>

Example:

# gazelle:repository com_github_pkg_errors github.com/pkg/errors

[jayconrod]

This directive should be written so that it affects import resolution for a specific language.

# gazelle:repository go com_github_pkg_errors github.com/pkg/errors

[jayconrod]

After #647, there is now a Repos field in config.Config which lists repositories read from the configuration file. # gazelle:repository should add fake rules to that list. That means the syntax should be rich enough to specify a rule kind and attributes. So something like this?

# gazelle:repository go_repository name=com_github_pkg_errors importpath=github.com/pkg/errors

It should be possible to override an actual repository with this rule. For example, if someone has a git_repository rule for org_golang_x_tools, they should be able to write:

# gazelle:repository go_repository name=org_golang_x_tools importpath=golang.org/x/tools

And Gazelle would proceed as if that rule were declared as a go_repository rule.