Update module spec (cosmos#5751)

docs/building-modules/structure.md

@@ -1,6 +1,8 @@
 <!--
 order: 12
-synopsis: This document outlines the recommended structure of Cosmos SDK modules. These ideas are meant to be applied as suggestions. Application developers are encouraged to improve upon and contribute to module structure and development design.
+synopsis: This document outlines the recommended structure of Cosmos SDK modules.These ideas are
+meant to be applied as suggestions. Application developers are encouraged to improve upon and
+contribute to module structure and development design.
 -->
 
 # Recommended Folder Structure
@@ -20,23 +22,24 @@ x/{module}
 │       └── tx.go
 ├── exported
 │   └── exported.go
-├── internal
-│   ├── keeper
-│   │   ├── invariants.go
-│   │   ├── keeper.go
-│   │   ├── ...
-│   │   └── querier.go
-│   └── types
-│       ├── codec.go
-│       ├── errors.go
-│       ├── events.go
-│       ├── expected_keepers.go
-│       ├── genesis.go
-│       ├── keys.go
-│       ├── msgs.go
-│       ├── params.go
-│       ├── ...
-│       └── querier.go
+├── keeper
+│   ├── invariants.go
+│   ├── keeper.go
+│   ├── ...
+│   └── querier.go
+├── types
+│   ├── codec.go
+│   ├── errors.go
+│   ├── events.go
+│   ├── expected_keepers.go
+│   ├── genesis.go
+│   ├── keys.go
+│   ├── msgs.go
+│   ├── params.go
+│   ├── types.pb.go
+│   ├── types.proto
+│   ├── ...
+│   └── querier.go
 ├── simulation
 │   ├── decoder.go
 │   ├── genesis.go
@@ -55,9 +58,8 @@ x/{module}
 - `alias.go`: The module's exported types, constants, and variables. These are mainly
 to improve developer ergonomics by only needing to import a single package. Note,
 there is nothing preventing developers from importing other packages from the module
-(excluding`internal/`) but it is recommended that `alias.go` have everything
-exposed that other modules may need. The majority of the exported values here will
-typically come from `internal/` (see below).
+but it is recommended that `alias.go` have everything exposed that other modules
+may need.
 - `client/`: The module's CLI and REST client functionality implementation and
 testing.
 - `exported/`: The module's exported types -- typically type interfaces. If a module
@@ -72,16 +74,10 @@ DRY and also alleviates import cycle chaos.
 - `genesis.go`: The module's genesis related business logic (e.g. `InitGenesis`).
 Note, genesis types are defined in `internal/types`.
 - `handler.go`: The module's message handlers.
-- `internal/`: The module's internal types and implementations. The purpose of
-this package is mainly two fold. First, it signals that this package is not
-intended to be used or imported anywhere outside the defining module. Secondly,
-it goes hand-in-hand with `alias.go` in that it allows public types and functions
-to be used internally while not being exposed outside to the outside world. This
-allows for greater freedom of development while maintaining API stability.
-  - `internal/types`: The module's type definitions such as messages, `KVStore`
-  keys, parameter types, and `expected_keepers.go` contracts.
-  - `internal/keeper`: The module's keeper implementation along with any auxiliary
-  implementations such as the querier and invariants.
+- `keeper/`: The module's keeper implementation along with any auxiliary
+implementations such as the querier and invariants.
+- `types/`: The module's type definitions such as messages, `KVStore` keys,
+parameter types, Protocol Buffer definitions, and `expected_keepers.go` contracts.
 - `module.go`: The module's implementation of the `AppModule` and `AppModuleBasic`
 interfaces.
 - `simulation/`: The module's simulation package defines all the required functions