Transform MavenLocal-based dependency to local project one

[alexander-yevsyukov]

Currently model-compiler/build.gradle contains the following code:

dependencies {
    // ...
    // The freshest version of the plugin required for tests
    protocPluginDependency = testCompileOnly "io.spine.tools:spine-protoc-plugin:$spineVersion@jar"
}

This means that we depend on the locally deployed JAR and to the plugin code in the project. It would be good to have only the dependency on locally built JARs.