clean up build.gradle files, move constants to buildSrc

buildSrc/build.gradle.kts

@@ -0,0 +1,11 @@
+plugins {
+    `kotlin-dsl`
+}
+
+repositories {
+    mavenCentral()
+}
+
+dependencies {
+    implementation("net.lingala.zip4j:zip4j:2.9.0")
+}

buildSrc/src/main/kotlin/Constants.kt

@@ -0,0 +1,33 @@
+import java.io.BufferedReader
+
+/*
+ * Copyright (C) Contributors to the Suwayomi project
+ * 
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+const val kotlinVersion = "1.5.21"
+
+const val MainClass = "suwayomi.tachidesk.MainKt"
+
+// should be bumped with each stable release
+val tachideskVersion = System.getenv("ProductVersion") ?: "v0.4.5"
+
+val webUIRevisionTag = System.getenv("WebUIRevision") ?: "r24"
+
+// counts commit count on master
+val tachideskRevision = runCatching {
+    System.getenv("ProductRevision") ?: Runtime
+        .getRuntime()
+        .exec("git rev-list HEAD --count")
+        .let { process ->
+            process.waitFor()
+            val output = process.inputStream.use {
+                it.bufferedReader().use(BufferedReader::readText)
+            }
+            process.destroy()
+            "r" + output.trim()
+        }
+}.getOrDefault("r0")
+