Enable sbt to specify config.resource without disabling gigahorse

Fix https://github.com/sbt/sbt/issues/3585.
Gigahorse.config tries to load specified config file in sbt's classpath
and of course fail to find the specified config file, and throw
java.io.IOException if we specify config.resource, because

We can avoid trying to load specified config file inside sbt by
avoid using Gigahorse.config that call `ConfigFactory.load()` inside.

This commit fixes librarymanagement not to use Gigahorse.config and make
it use gigahorse.Config() which returns gigahorse's default config
without calling `ConfigFactory.load()` instead.

core/src/main/scala/sbt/librarymanagement/Http.scala

@@ -3,5 +3,5 @@ package sbt.librarymanagement
 import gigahorse._, support.okhttp.Gigahorse
 
 object Http {
-  lazy val http: HttpClient = Gigahorse.http(Gigahorse.config)
+  lazy val http: HttpClient = Gigahorse.http(gigahorse.Config())
 }

ivy/src/main/scala/sbt/internal/librarymanagement/ivyint/GigahorseUrlHandler.scala

@@ -214,7 +214,7 @@ object GigahorseUrlHandler {
 
   private val EmptyBuffer: Array[Byte] = new Array[Byte](0)
 
-  lazy val http: HttpClient = Gigahorse.http(Gigahorse.config)
+  lazy val http: HttpClient = Gigahorse.http(gigahorse.Config())
 
   private lazy val okHttpClient: OkHttpClient = {
     http