Incorporate Magritte approach that avoids having to copy write-version.gradle

[eshlin]

@iamdanfox mentioned that Magritte has an approach that avoids the need to copy over write-version.gradle. It would be great to incorporate this into the plugin so users of dropwizard-version-info don't have to copy over the the write-version.gradle file.

"We’ve actually ended up with a way nicer way of doing version stuff in Magritte
we add a sourceSet called generated_resources, which gradle puts the version file in
It’s might be quite easy to pluginize and should eliminate the need to copy the write-version.gradle file"

[iamdanfox]

This approach involves adding a source set to main:

sourceSets {
  main {
    output.dir("$buildDir/generated_resources/main", builtBy: 'generateVersionFile')
  }
}

This task then just needs to write the file somehow...

task generateVersionFile {
  def versionFile = file("$buildDir/generated_resources/main/version.properties")
  outputs.file versionFile
  inputs.properties version: version // necessary to make task refresh properly

  doLast {
    versionFile.text = "version: ${version}"
  }
}

I think it might be neater to express this a Copy task... as usual, there are a million ways to do everything.

Upsides of this approach: it will work if you run ./gradlew eclipse or ./gradlew idea, as well as ./gradlew compileJava.