fix(build): Don't replace extern_paths (#261)

* don't replace extern_path'd paths

tonic-build will replace all paths except for google well known types
with one rooted at the super module. For p

fix(build): Don't replace extern_paths (#261)

* don't replace extern_path'd paths

tonic-build will replace all paths except for google well known types
with one rooted at the super module. For paths that have already been
replaced with a fully qualified path via the extern_path config option,
(e.g. "::uuid::Uuid"), this results in an invalid path
(e.g. "super::::uuid::Uuid"), and a build failure. These paths should
also be excluded when prefixing relative modules with super.

* add doc in tonic-build clarifying extern_path

extern_path expects fully qualified proto and rust paths

* add test cast case for extern path fix

add a test that extern path does indeed result in service types using
the specified type from an external crate. we test this by creating a
service type that has a proto from a different crate, and asserting that
it does indeed impl a trait from that crate

* add license/publish to extern_path test crates

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