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 the docs/howto/separate-models-file.md file.

Main code changes made in this PR

• Added optional output_models_package, models_package_import_path and output_query_files_directory options to gen/go configuration. (internal/codegen/golang/opts/options.go)
• Added Package field to Struct type to specify the correct type of the Struct since they can be in different package now. (internal/codegen/golang/struct.go and internal/codegen/golang/result.go)
• Type() method of QueryValue will return {Package}.{Name} for Struct types if the Package field of the Struct is not empty. (internal/codegen/golang/query.go)
• Added {Package}. prefix to enum types if configuration specifies separate models package. (changed only internal/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 my sqlc-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.