Move some more build logic to buildSrc

Look, I don't enjoy having 600 LOC long build.gradle files, it's just
very easy to do! This at least moves some of the complexity elsewhere,
so the build script is a little more declarative.

buildSrc/src/main/kotlin/cc/tweaked/gradle/ProcessHelpers.kt

@@ -0,0 +1,34 @@
+package cc.tweaked.gradle
+
+import org.codehaus.groovy.runtime.ProcessGroovyMethods
+import java.io.BufferedReader
+import java.io.IOException
+import java.io.InputStreamReader
+
+internal object ProcessHelpers {
+    fun startProcess(vararg command: String): Process {
+        // Something randomly passes in "GIT_DIR=" as an environment variable which clobbers everything else. Don't
+        // inherit the environment array!
+        return Runtime.getRuntime().exec(command, arrayOfNulls(0))
+    }
+
+    fun captureOut(vararg command: String): String {
+        val process = startProcess(*command)
+        val result = ProcessGroovyMethods.getText(process)
+        if (process.waitFor() != 0) throw IOException("Command exited with a non-0 status")
+        return result
+    }
+
+    fun captureLines(vararg command: String): List<String> {
+        return captureLines(startProcess(*command))
+    }
+
+    fun captureLines(process: Process): List<String> {
+        val out = BufferedReader(InputStreamReader(process.inputStream)).use { reader ->
+            reader.lines().filter { it.isNotEmpty() }.toList()
+        }
+        ProcessGroovyMethods.closeStreams(process)
+        if (process.waitFor() != 0) throw IOException("Command exited with a non-0 status")
+        return out
+    }
+}