Extract install_sdk and install_tool from install. NFC (#432)

This change simply splits this function into its two logical parts.

emsdk.py

@@ -1842,7 +1842,13 @@ class Tool(object):
       return False
 
     if self.id == 'sdk':
+      return self.install_sdk()
+    else:
+      return self.install_tool()
+
+  def install_sdk(self):
     print("Installing SDK '" + str(self) + "'..")
+
     for tool_name in self.uses:
       tool = find_tool(tool_name)
       if tool is None:
@@ -1856,9 +1862,11 @@ class Tool(object):
       success = emscripten_npm_install(self, os.path.join(emsdk_path(), install_path, 'emscripten'))
       if not success:
         return False
+
     print("Done installing SDK '" + str(self) + "'.")
     return True
-    else:
+
+  def install_tool(self):
     # We should not force reinstallation of python if it already exists, since that very python
     # may be interpreting the current emsdk.py script we are executing. On Windows this would
     # lead to a failure to uncompress the python zip file as the python executable files are in use.
@@ -1868,6 +1876,7 @@ class Tool(object):
     if self.id == 'python' and self.is_installed():
       print("Skipped installing " + self.name + ", already installed.")
       return True
+
     print("Installing tool '" + str(self) + "'..")
     url = self.download_url()
 
@@ -1921,6 +1930,7 @@ class Tool(object):
     if not success:
       print("Installation failed!")
       return False
+
     print("Done installing tool '" + str(self) + "'.")
 
     # Sanity check that the installation succeeded, and if so, remove unneeded