Cannot compile a Scrooge Finagle service

Hello,

Since I upgraded my Scala rules (last version known to work was 21c8913a2cf35130870df9f07a781afc77f33693, from last March), I cannot compile anymore my Thrift services with Scrooge. Creating `scrooge_scala_library` when the IDL file contains only structs is ok, but I cannot create a library when the IDL file also contains a service.

When trying to compile the target `scrooge_scala_library`, I get a lot of compilation errors looking like:
```error: object finagle is not a member of package com.twitter```

Previously, I used to add "//3rdparty/jvm/com/twitter:finagle_thrift" to my `deps` to bring in the Finagle dependency, but if I do so, I now get another error:
``` (rule 'scala_import') doesn't contain declared provider 'ThriftInfo'```

I couldn't spot any documentation or tests about compiling Finagle Scrooge services, so I am a bit stuck. I can contribute tests/documentation if given a hint about how things are supposed to work. Thanks!

Start by reproducing the scrooge_scala_library target with an IDL containing a service, then inspect the Scala rules and the dependency declarations for //3rdparty/jvm/com/twitter:finagle_thrift. Done means the service target compiles without the missing finagle or ThriftInfo errors, with tests or documentation covering the supported setup.

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