Use custom dependency resolution

- Use Apache Ivy to resolve artifact URLs
- Update build model with full artifact IDs
- Generate Maven module metadata to support dynamic version constraints
- Resolve snapshot versions and generate snapshot metadata
- Add test fixtures and rewrite Gradle plugin tests
- Update dependencies

build.gradle.kts

@@ -3,15 +3,20 @@ plugins {
     idea
     kotlin("jvm") version embeddedKotlinVersion apply false
     kotlin("kapt") version embeddedKotlinVersion apply false
-    id("com.github.johnrengelman.shadow") version "5.1.0" apply false
-    id("org.ajoberstar.stutter") version "0.5.0" apply false
+    id("com.github.johnrengelman.shadow") version "5.2.0" apply false
+    id("org.ajoberstar.stutter") version "0.5.1" apply false
 }
 
+group = "org.nixos.gradle2nix"
+version = property("VERSION") ?: "unspecified"
+
 subprojects {
     repositories {
         jcenter()
         maven { url = uri("https://repo.gradle.org/gradle/libs-releases") }
     }
+    group = rootProject.group
+    version = rootProject.version
 }
 
 allprojects {
@@ -45,7 +50,7 @@ allprojects {
 
 tasks {
     wrapper {
-        gradleVersion = "5.6.3"
+        gradleVersion = "6.1"
         distributionType = Wrapper.DistributionType.ALL
     }
 }