Support circular dependency in dfx build

[chenyan-dfinity]

Currently, if there is a circular dependency in dfx.json, dfx build will abort with The dependency analyzer failed: Found circular dependency: backend.

Since we create all canister ids first, we should be able to handle circular dependency. This is useful for access control, e.g. backend will only respond to calls from the frontend, and not from other principals.

In Motoko playground, the backend can create new canisters with its own cycles. We want to prevent other people from calling this API, so that people cannot create canisters for free using this API.