Do less allocation when building routes.

[rylev]

In particular avoid allocating duplicate routes if they won't be used.

This will avoid allocating:

• 1 vector of routes_iter
• An effective_id string for every route (only allocating if there is a duplicate and duplicates will be used)
• A component_id string for every duplicate route (only allocating if duplicates will be used)

This will be particularly useful for embedders that need to build routes for often than Spin CLI does.