Make `local:` snap locations be actual URIs

Right now `local:` locations look like this: `local:http://localhost/`, which is not a valid URI.

We should try to normalise Snap Locations to all be URIs and make them look like this: `local://localhost/`, or even better - just `http://localhost/` with restrictions on hostname.

Context:
1. https://github.com/MetaMask/snaps-monorepo/pull/1019#discussion_r1039523629
2. [SIP-8](https://github.com/MetaMask/SIPs/blob/main/SIPS/sip-8.md)

Start by reading the linked PR discussion and SIP-8 to resolve whether the format should be `local://localhost/` or an HTTP URI with hostname restrictions. Then locate the Snap Location handling and its tests; done means locations are normalized as valid URIs and the agreed restrictions are covered by tests.

Written by the indexing model from the issue text.