make Services a map[string]ServiceConfig to reflect yaml structure

[ndeloof]

the docker/cli compose parser we inherited as compose-go used a []ServiceConfig to map to yaml services. I'm not sure what was the reason for this design decision, and propose we adopt a natural map[string]ServiceConfig type.
For convenience, ServiceConfig.Name is set to map key

[milas]

LGTM, just one point of concern where we're returning p.Services directly

Beyond that, we should think about our iteration patterns to avoid weirdness from random Go map iteration order

[milas]

I think we should make a copy of the map here – if a caller ends up mutating the result, they'll be unintentionally changing the project

[ndeloof]

This issue we already have with networks/volumes/secrets/configs
A global "immutability" refactoring is under consideration, but will come as a future effort
Please note if we try to encapsulate the map nature of this, we will have to manage the yaml unmarshaling, so we could consider to offer "mutation-safe" iterator Services.Each(func(name, ServiceConfig)), but won't preserve third-party to use a plain range. Please also note mutating the model could be required by user