Gradle cleanup/overhaul.

- Fix a whole bunch of ErrorProne warnings. It's still quite noisy, but
   most of the remaining ones now come from Java.

 - Move some config from java-conventions to a new base-conventions,
   which can be used for non-Java projects. As part of this, we now kill
   the CCTweaked plugin, and just install the cct extension in
   base-conventions instead.

 - Kill inlineProject/externalSources. We now:
    - Add an embeddedProject configuration which is used to define
      projects that are embedded within the final jar. We do some fun
      variant resolution in ExternalProjectArtifacts to extract the
      class/resource/source directories from the configuration.
    - Add a new verifyClassesMatch task which recompiles the game and
      ensures compiling everything against the loader's version of
      Minecraft gives us the same result.

projects/web/src/main/java/dan200/computercraft/core/apis/http/request/THttpRequest.java

@@ -5,6 +5,7 @@
 package dan200.computercraft.core.apis.http.request;
 
 import cc.tweaked.web.Main;
+import com.google.common.base.Splitter;
 import dan200.computercraft.core.Logging;
 import dan200.computercraft.core.apis.IAPIEnvironment;
 import dan200.computercraft.core.apis.handles.ArrayByteChannel;
@@ -122,7 +123,7 @@ public class THttpRequest extends Resource<THttpRequest> {
         var reader = new ReadHandle(contents, binary);
 
         Map<String, String> responseHeaders = new HashMap<>();
-        for (var header : request.getAllResponseHeaders().split("\r\n")) {
+        for (var header : Splitter.on("\r\n").split(request.getAllResponseHeaders())) {
             var index = header.indexOf(':');
             if (index < 0) continue;