Allow API to reside at submodule of crate

[jayvdb]

Split off from #638

Potentially crate_name args should be renamed.

[ahl]

Is this fixing a bug or is this adding some new functionality? I'm not sure I understand the utility.

[jayvdb]

This is new functionality, needed in order to have the generated cli & httpmock be a module that resides beside the generated client.

Currently the generator uses format_ident!(..) which only supports foo - this PR allows it to support foo::bar.

It is needed for #638 , where the types+client, mocks and cli are three separate generated modules, and lib.rs is a manually authored file that combines those generated files and potentially adds other human authored functionality. The top level lib.rs does not contain the types+client. Therefore the mocks and cli need to be told where to find the types+client within the crate.

@franklin-ai is using this approach to be able to greater control over our crate lib.rs, re-exporting at the top level only a subset of the generated mods, and including lots of other human written code, such as impls on the generated types.

[jayvdb]

ping @ahl . there are a bunch of other improvements which depend on this, some with a PR, others waiting on it getting merged.

[ahl]

good enough; thanks