Avoid re-downloading files that are already downloaded (#460)

* Writes a .emsdk_version file to output directories using the name (which contains a version number and/or hash) once all installation steps have completed successfully.
* If that file exists, skip downloading/installation hooks.

emsdk.py

@@ -1885,6 +1885,14 @@ class Tool(object):
       print("Skipped installing " + self.name + ", already installed.")
       return True
 
+    version_id = self.name
+    version_file_path = os.path.join(self.installation_path(), '.emsdk_version')
+    if os.path.isfile(version_file_path):
+      with open(version_file_path, 'r') as version_file:
+        if version_id == version_file.read():
+          print("Skipped installing " + self.name + ", already installed.")
+          return True
+
     print("Installing tool '" + str(self) + "'..")
     url = self.download_url()
 
@@ -1945,6 +1953,8 @@ class Tool(object):
     # leftover installation files.
     if self.is_installed():
       self.cleanup_temp_install_files()
+      with open(version_file_path, 'w') as version_file:
+        version_file.write(version_id)
     else:
       print("Warning: The installation of '" + str(self) + "' seems to have failed, but no error was detected. Either something went wrong with the installation, or this may indicate an internal emsdk error.")
     return True