Move repo name validation into Repo

Move repo name validation into Repo

The validate_repo_name free function is now Repo::validate_name, a
private method. Repo gains two constructors: new(base, name) for
name-based construction and from_path(base, path) for path-based
construction. Quire::repo and Quire::repo_from_path delegate to these.

Assisted-by: GLM-5.1 (zai/glm-5.1)