Publish separate scip-java-proto artifact with scip.proto generated classes

Fixes #522

Previously, it was necessary to generate classes from the scip.proto source
if you wanted to write Java tooling that consumes SCIP. This commit
reorganizes the build so that we publish a new
com.sourcegraph:scip-java-proto artifact that only includes the
generated protobuf classes from scip.proto. Note that we don't guarantee
binary compatibility for public classes in this module.

Author: olafurpg

build.sbt

@@ -126,10 +126,10 @@ lazy val plugin = project
   )
   .dependsOn(semanticdb)
 
-lazy val scip = project
-  .in(file("scip-semanticdb"))
+lazy val scipProto = project
+  .in(file("scip-java-proto"))
   .settings(
-    moduleName := "scip-semanticdb",
+    moduleName := "scip-java-proto",
     javaToolchainVersion := "8",
     javaOnlySettings,
     libraryDependencies +=
@@ -138,7 +138,15 @@ lazy val scip = project
       Seq(PB.gens.java(V.protobuf) -> (Compile / sourceManaged).value),
     Compile / PB.protocOptions := Seq("--experimental_allow_proto3_optional")
   )
-  .dependsOn(semanticdb)
+
+lazy val scip = project
+  .in(file("scip-semanticdb"))
+  .settings(
+    moduleName := "scip-semanticdb",
+    javaToolchainVersion := "8",
+    javaOnlySettings
+  )
+  .dependsOn(semanticdb, scipProto)
 
 lazy val cli = project
   .in(file("scip-java"))