Fix #2875: Remove JSEnv.loadLibs, resolvedJSEnv and loadedJSEnv.

We remove `resolvedJSEnv` simply by initializing `jsEnv` by default
in the project scope, and using `jsEnv` instead of `resolvedJSEnv`.

We remove `loadedJSEnv` by using instead the pair
`jsEnv` + `jsExecutionFiles` explicitly.

Finally, we can remove `JSEnv.loadLibs`, as it was only useful for
`loadedJSEnv`.

Author: sjrd

sbt-plugin-test/project/Jetty9Test.scala

@@ -21,7 +21,8 @@ object Jetty9Test {
   private val jettyPort = 23548
 
   val runSetting = run <<= Def.inputTask {
-    val jsEnv = (loadedJSEnv in Compile).value.asInstanceOf[ComJSEnv]
+    val env = (jsEnv in Compile).value.asInstanceOf[ComJSEnv]
+    val files = (jsExecutionFiles in Compile).value
     val jsConsole = scalaJSConsole.value
 
     val code = new MemVirtualJSFile("runner.js").withContent(
@@ -42,7 +43,7 @@ object Jetty9Test {
       """
     )
 
-    val runner = jsEnv.comRunner(code :: Nil)
+    val runner = env.comRunner(files :+ code)
 
     runner.start(streams.value.log, jsConsole)