x/tools/go/packages: document Package fields that should be populated by DriverResponse

[findleyr]

https://pkg.go.dev/golang.org/x/tools/go/packages#DriverResponse documents that packages should be returned by go/packages drivers, but is unclear about precisely which fields must be populated. (Clearly, some fields cannot be json serialized). There is some discussion of syntax, types, and imports, but this discussion does not enumerate fields.

We should document, perhaps at the field level, which fields are relevant for the driver protocol.

[gopherbot]

Change https://go.dev/cl/588141 mentions this issue: go/packages: document fields that are part of JSON schema