@param [String] requirement_name the spec name to search for @return [Object] the locked spec named ‘requirement_name`, if one
is found on {#base}
Adds a new {Edge} to the dependency graph without checking for circularity.
Finds a spec and the source_uri it came from for gem gem_name and version. Returns an Array of specs and sources required for installation of the gem.
@return [String] the name of the source of ‘locked’ dependencies, i.e.
those passed to {Resolver#resolve} directly as the `base`