Display warning if root state property has the same name with module

[hootlex]

What problem does this feature solve?

If there is a root state property with the same name of a module, the module is prioritized. Right now there is no warning.

Example:

// store
modules: {
    users
},
state: {
    users: []
}

store.state.users points to the users module and users state property is shallowed

What does the proposed API look like?

There should be a warning pointing out the mistake and intriguing the user to rename the state property.

[winniehell]

A similar warning has been implemented in #1554.