fix(repository): build relations based on their names

[raymondfeng]

The current implementation uses property names as the key of
relations and it's not compatible with legacy juggler.

See #1909

Checklist

• npm test passes on your machine
• New tests added or existing tests modified to cover all changes
• Code conforms with the style guide
• API Documentation in code was updated
• Documentation in /docs/site was updated
• Affected artifact templates in packages/cli were updated
• Affected example projects in examples/* were updated

[bajtos]

I am not able to fully comprehend the ramification of proposed implementation changes. But since I don't see any obvious problem and the changes in the test suite looks reasonable, the patch LGTM 👍

[raymondfeng]

I realized this PR is just a starting point to fully support include. There are a few issues:

1. LB4 adds a property for hasMany relation. For example: A customer class will have orders property but loopback-datasource-juggler does not like that for relational databases
2. We don't convert LB4 relations into juggler model definition - see https://github.com/strongloop/loopback-next/blob/master/packages/repository/src/repositories/legacy-juggler-bridge.ts#L131. Mapping LB4 relations into juggler is difficult as it needs to remap the source/target model too.