Migrate GradleInspector's custom plugin to github-dependency-graph-gradle-plugin

[sschuberth]

Instead of maintain our own Gradle plugin, it should be pretty much straight forward to migrate to the official github-dependency-graph-gradle-plugin instead, applying SimpleDependencyGraphPlugin and running the ForceDependencyResolutionPlugin_resolveAllDependencies task similar to

// Running this like
//
// $ ./gradlew -I ~/Downloads/github-dependency-graph-gradle-plugin.init.gradle ForceDependencyResolutionPlugin_resolveAllDependencies
//
// creates these two files:
//
// - ./build/reports/dependency-graph-snapshots/dependency-graph.json
// - ./build/reports/dependency-graph-snapshots/dependency-list.txt

initscript {
    repositories {
        maven {
            url = uri("https://plugins.gradle.org/m2/")
        }
    }

    dependencies {
        classpath("org.gradle:github-dependency-graph-gradle-plugin:1.2.2")
    }
}

apply plugin: org.gradle.dependencygraph.simple.SimpleDependencyGraphPlugin

The only blocker I currently see might be gradle/github-dependency-graph-gradle-plugin#42, which would be needed to solve #7995.