Assign Package.prototype.mainModulePath eagerly for preloaded packages

Many packages currently use this instance variable instead of calling
`Package.prototype.getMainModulePath`. With this commit we will eagerly
compute it for preloaded packages too and therefore prevent third party
packages that rely on this implementation detail from breaking.

src/package.coffee

@@ -102,6 +102,10 @@ class Package
       @path = path.join(@packageManager.resourcePath, @path)
       @loadStylesheets()
 
+      # Unfortunately some packages are accessing `@mainModulePath`, so we need
+      # to compute that variable eagerly also for preloaded packages.
+      @getMainModulePath()
+
   load: ->
     @measure 'loadTime', =>
       try