Fixes#835

This PR adds exporting models to a different package feature to Go code generator.

I also added a how-to page for this feature, you can check thedocs/howto/separate-models-file.md file.

Main code changes made in this PR

• Added optionaloutput_models_package,models_package_import_path andoutput_query_files_directory options to gen/go configuration. (internal/codegen/golang/opts/options.go)
• AddedPackage field toStruct type to specify the correct type of the Struct since they can be in different package now. (internal/codegen/golang/struct.go andinternal/codegen/golang/result.go)
• Type() method ofQueryValue will return{Package}.{Name} forStruct types if thePackage field of theStruct is not empty. (internal/codegen/golang/query.go)
• Added{Package}. prefix to enum types if configuration specifies separate models package. (changed onlyinternal/codegen/golang/postgresql_type.go since only postgresql implementation supports typed enum values)
• Deciding whether the generated file needs to import the separated models file package. (internal/codegen/golang/imports.go)

Note for sqlc users

You can simply try this feature using mysqlc-gen-go plugin fork. I will try to keep that plugin and this PR in sync. Surely sqlc-gen-go plugin configuration is not same as the sqlc gen configuration but the feature implementation code is the same as this PR.