Rewrite download utils (#681)

- eTag support (It seems Mojang's CDN and our meta/maven do not support this right now)
- Age based caching.
- Sha1 based caching.
- HTTP(S) proxy settings should now be respected.
- The downloader has better awareness of offline mode and refresh deps, cleaning up the calling code a bit.
- Uses the new Java 11 HTTP client, provides async support for downloading multiple files
- Progress handling (TODO needs hooking up to gradle)
- Better compression support. Handled by [Methanol](https://mizosoft.github.io/methanol/) 
- Unit tested with a real web server.

src/main/java/net/fabricmc/loom/extension/LoomGradleExtensionImpl.java

@@ -24,6 +24,7 @@
 
 package net.fabricmc.loom.extension;
 
+import java.net.URISyntaxException;
 import java.nio.file.Path;
 import java.util.ArrayList;
 import java.util.List;
@@ -48,6 +49,8 @@ import net.fabricmc.loom.configuration.providers.mappings.MappingsProviderImpl;
 import net.fabricmc.loom.configuration.providers.minecraft.MinecraftProvider;
 import net.fabricmc.loom.configuration.providers.minecraft.mapped.IntermediaryMinecraftProvider;
 import net.fabricmc.loom.configuration.providers.minecraft.mapped.NamedMinecraftProvider;
+import net.fabricmc.loom.util.download.Download;
+import net.fabricmc.loom.util.download.DownloadBuilder;
 
 public class LoomGradleExtensionImpl extends LoomGradleExtensionApiImpl implements LoomGradleExtension {
 	private final Project project;
@@ -206,9 +209,36 @@ public class LoomGradleExtensionImpl extends LoomGradleExtensionApiImpl implemen
 		transitiveAccessWideners.addAll(accessWidenerFiles);
 	}
 
+	@Override
+	public DownloadBuilder download(String url) {
+		DownloadBuilder builder;
+
+		try {
+			builder = Download.create(url);
+		} catch (URISyntaxException e) {
+			throw new RuntimeException("Failed to create downloader for: " + e);
+		}
+
+		if (project.getGradle().getStartParameter().isOffline()) {
+			builder.offline();
+		}
+
+		if (project.getGradle().getStartParameter().isRefreshDependencies() || Boolean.getBoolean("loom.refresh")) {
+			builder.forceDownload();
+		}
+
+		// TODO
+		//builder.executor();
+
+		return builder;
+	}
+
 	@Override
 	protected <T extends IntermediateMappingsProvider> void configureIntermediateMappingsProviderInternal(T provider) {
 		provider.getMinecraftVersion().set(getProject().provider(() -> getMinecraftProvider().minecraftVersion()));
 		provider.getMinecraftVersion().disallowChanges();
+
+		provider.getDownloader().set(this::download);
+		provider.getDownloader().disallowChanges();
 	}
 }