How Can I Write A Dependency List On The Fly - groovy

Question

I occasionally get code drops from a third party as a zip file of jars that I need to use as dependencies.
I want to unzip the jars into a flatDir repository and update a dependencies.gradle file automatically. I can do the unzip just fine, the writing to the file is the tricky bit.
Lets give an example using open source libraries:
If the dependencies.gradle originally looked like this:
repositories {
flatDir {
dirs 'lib'
}
}
dependencies {
compile 'commons-codec:1.2'
compile 'commons-logging:1.2'
compile 'log4j:1.2.17'
}
Then a Zip file arrived containing commons-codec-1.10.jar, commons-logging-1.2.jar and guava-18.0.jar. The dependencies.gradle would have to be updated to:
repositories {
flatDir {
dirs 'lib'
}
}
dependencies {
compile 'commons-codec:1.10'
compile 'commons-logging:1.2'
compile 'log4j:1.2.17'
compile 'guava:18.0'
}
I want to have specific versions and not just glob the whole flatDir as there might be different versions of the same library in the flatDir.
I can't use normal filtering as the # symbol wont be in the file.
The challenge is to recognise when a dependency already exists and needs updating, when a dependency already exists and doesn't need updating and when a dependency doesn't exist and needs adding.
I am using a Copy task with a zipTree to unzip so my thought is to use eachFile with a closure to do the work. However, I'm not quite sure how to do the steps outlined in the paragraph above.