refactor: move xmodule folder to root

- Moving xmodule folder to root as we're dissolving sub-projects of common folder in edx-platform
    - More info: https://openedx.atlassian.net/browse/BOM-2579
- -e common/lib/xmodule has been removed from the requirements as xmodule has itself become the part of edx-platform and not being installed through requirements
- The test files common/lib/xmodule/test_files/ have been removed as they are not being used anymore

xmodule/exceptions.py

@@ -0,0 +1,54 @@
+# lint-amnesty, pylint: disable=missing-module-docstring
+class InvalidDefinitionError(Exception):
+    pass
+
+
+class NotFoundError(Exception):
+    pass
+
+
+class ProcessingError(Exception):
+    '''
+    An error occurred while processing a request to the XModule.
+    For example: if an exception occurs while checking a capa problem.
+    '''
+    pass  # lint-amnesty, pylint: disable=unnecessary-pass
+
+
+class InvalidVersionError(Exception):
+    """
+    Tried to save an item with a location that a store cannot support (e.g., draft version
+    for a non-leaf node)
+    """
+    def __init__(self, location):
+        super().__init__()
+        self.location = location
+
+
+class SerializationError(Exception):
+    """
+    Thrown when a module cannot be exported to XML
+    """
+    def __init__(self, location, msg):
+        super().__init__(msg)
+        self.location = location
+
+
+class UndefinedContext(Exception):
+    """
+    Tried to access an xmodule field which needs a different context (runtime) to have a value.
+    """
+    pass  # lint-amnesty, pylint: disable=unnecessary-pass
+
+
+class HeartbeatFailure(Exception):
+    """
+    Raised when heartbeat fails.
+    """
+
+    def __init__(self, msg, service):
+        """
+        In addition to a msg, provide the name of the service.
+        """
+        self.service = service
+        super().__init__(msg)