Support specifying main module in package.cson

Migrated strip-trailing-whitespace package to include
a package.cson with a main being a file in the lib directory
instead of the index.coffee at the root of the package.

src/app/atom-package.coffee

@@ -16,11 +16,19 @@ class AtomPackage extends Package
       @loadMetadata()
       @loadKeymaps()
       @loadStylesheets() if @autoloadStylesheets
-      rootView?.activatePackage(@name, this) if require.resolve(@path)
+      if packageMain = @getPackageMain()
+        rootView?.activatePackage(@name, packageMain)
     catch e
       console.warn "Failed to load package named '#{@name}'", e.stack
     this
 
+  getPackageMain: ->
+    mainPath = require.resolve(@metadata.main) if @metadata.main
+    if mainPath
+      require(mainPath)
+    else if require.resolve(@path)
+      this
+
   loadMetadata: ->
     if metadataPath = fs.resolveExtension(fs.join(@path, "package"), ['cson', 'json'])
       @metadata = fs.readObject(metadataPath)

src/packages/strip-trailing-whitespace/lib/strip-trailing-whitespace.coffee

@@ -1,7 +1,4 @@
-AtomPackage = require 'atom-package'
-
 module.exports =
-class StripTrailingWhitespace extends AtomPackage
   activate: (rootView) ->
     rootView.eachBuffer (buffer) => @stripTrailingWhitespaceBeforeSave(buffer)
 

src/packages/strip-trailing-whitespace/package.cson

@@ -0,0 +1 @@
+'main': 'strip-trailing-whitespace/lib/strip-trailing-whitespace'