Namespacing support

Currently relay-compiler enforces a very strict naming convention:

`Parse error: Error: RelayFindGraphQLTags: Container fragment names must be _`.

Which makes sense in small apps or apps where every component easily can have a unique name, but in bigger apps that is not realistic - which is why namespaces were invented. Imagine having two UserDetails components, one for the public part of the site and one for the Manager/logged-in part of the site - with relay you would be forced to call them something different.

**My old workaround, that doesn´t work with the latest relay**

I used to be able to append an ugly namespace component to the query/mutation/fragment names:

`fragment UserDetailsMain...`

But now that also doesn´t work anymore.

**How would I solve this?**

Simple relaxing the naming of queries, mutations and fragments to prefix the module name with something _including_ underscores and/or dots would allow for easy namespacing support:

`Manager.UserDetails` / `Main.UserDetails`

Making this automatic in the future would be even better.

Start by reading relay-compiler's naming validation and its existing tests, then compare the current restrictions with the proposed Manager.UserDetails and Main.UserDetails forms. Done means the compiler accepts the intended names without breaking query, mutation, or fragment validation, with tests covering namespaced and invalid forms.

Written by the indexing model from the issue text.