Add a map to `Project` that has Compose YAML filename as key and
any `IncludeConfig`s loaded from it.

This is populated as the project is recursively loaded.

For example:
```
  proj/
    compose.yaml
    a/
      compose.yaml
    b/
      compose.yaml
```

If `project/compose.yaml` has:
```
include: ['./a/compose.yaml']
```

And `project/a/compose.yaml` has:
```
include: ['../b/compose.yaml']
```

The final result after load is conceptually:
```
{
  "proj/compose.yaml": ["proj/a/compose.yaml"],
  "proj/a/compose.yaml": ["proj/b/compose.yaml"],
}
```
(Note: in reality, it's a list of `IncludeConfig`, which has multiple
fields. Example is simplified.)

Relative path resolution is based on overall loader configuration,
but note that disabling it does not work properly for `include`
currently due to other issues.

This makes it possible for the caller to understand a bit more
about the loaded project resources. We're really overdue for a bit
of an overhaul/refactor of the loader - at that point, I think it
might be better to have a `Loader` object type that can track stuff
like this on the instance because it's weirdly both part of the
project and NOT part of the project at the moment (similar to
`Profiles`, project name, etc).

Signed-off-by: Milas Bowman <milas.bowman@docker.com>