Update load-plugin

Use `if-absent` so that we don't load the plugins and apply
transformations to the build if this is not necessary.

Author: Duhemm

vscode-dotty/package.json

@@ -51,7 +51,7 @@
     "vscode:prepublish": "npm install && ./node_modules/.bin/tsc -p ./",
     "compile": "./node_modules/.bin/tsc -p ./",
     "test": "node ./node_modules/vscode/bin/test",
-    "postinstall": "node ./node_modules/vscode/bin/install && curl -L -o out/coursier https://github.com/coursier/coursier/raw/v1.0.0/coursier && curl -L -o out/load-plugin.jar https://github.com/scalacenter/load-plugin/releases/download/v0.1.0/load-plugin_2.12-0.1.0.jar"
+    "postinstall": "node ./node_modules/vscode/bin/install && curl -L -o out/coursier https://github.com/coursier/coursier/raw/v1.0.0/coursier && curl -L -o out/load-plugin.jar https://oss.sonatype.org/content/repositories/releases/ch/epfl/scala/load-plugin_2.12/0.1.0+2-496ac670/load-plugin_2.12-0.1.0+2-496ac670.jar"
   },
   "extensionDependencies": [
     "daltonjorge.scala"

vscode-dotty/src/extension.ts

@@ -94,15 +94,21 @@ function configureIDE() {
     title: 'Configuring IDE...'
   }, (progress) => {
 
+      const applyLoadPlugin = "apply -cp " + loadPluginPath + " ch.epfl.scala.loadplugin.LoadPlugin"
+      const ifAbsentCommands = [
+        "if-absent dotty.tools.sbtplugin.DottyPlugin",
+        "\"set every scalaVersion := \\\"0.8.0-bin-SNAPSHOT\\\"\"",
+        "\"load-plugin ch.epfl.lamp:sbt-dotty:0.2.0-SNAPSHOT dotty.tools.sbtplugin.DottyPlugin\"",
+        "\"load-plugin ch.epfl.lamp:sbt-dotty:0.2.0-SNAPSHOT dotty.tools.sbtplugin.DottyIDEPlugin\""
+      ].join(" ")
+
     const sbtPromise =
       cpp.spawn("java", [
           "-jar", coursierPath,
           "launch",
           "org.scala-sbt:sbt-launch:1.1.2", "--",
-          "apply -cp " + loadPluginPath + " ch.epfl.scala.loadplugin.LoadPlugin",
-          "set every scalaVersion := \"0.8.0-bin-SNAPSHOT\"",
-          "load-plugin ch.epfl.lamp:sbt-dotty:0.2.0-SNAPSHOT dotty.tools.sbtplugin.DottyPlugin",
-          "load-plugin ch.epfl.lamp:sbt-dotty:0.2.0-SNAPSHOT dotty.tools.sbtplugin.DottyIDEPlugin",
+          applyLoadPlugin,
+          ifAbsentCommands,
           "configureIDE"
       ])
     const sbtProc = sbtPromise.childProcess