Map messages should not be child messages

[coryan]

Map fields are represented as (synthetic) child messages of the message that contains the field. Then in the Mustache templates we need to ignore them:

google-cloud-rust/generator/internal/rust/templates/common/message.mustache

Line 16 in 4f1dadc

{{^IsMap}}

We should consider adding these messages to the MessageById table, but not to the child messages. We can simplify the mustache templates, and avoid thinking about these so much.