use fully qualified names for Go types

[rogpeppe]

We're currently only using the base Go type name as the Avro name, but this can lead to name clashes if there are two types of the same name from different packages.
We could instead use a fully qualified name derived in some way (as-yet-to-be-specified) from the Go package path.

[kishansairam9]

can this be prioritised, this is blocking our usage of this library and falling back to https://github.com/actgardner/gogen-avro, which is no longer maintained at the moment. also having the "short" mode like in gogen would be a great addition