upgrade: drop the prefix to the 'allowednewrequirements' function Now that we are in the 'upgrade' module we can simplify the name.

upgrade: drop the prefix to the 'improvement' class Now that we are in the 'upgrade' module we can simplify the name.

upgrade: drop the prefix to the 'supportremovedrequirements' function Now that we are in the 'upgrade' module we can simplify the name.

upgrade: drop the prefix to the 'blocksourcerequirements' function Now that we are in the 'upgrade' module we can simplify the name.

upgrade: drop the prefix to the 'requiredsourcerequirements' function Now that we are in the 'upgrade' module we can simplify the name.

upgrade: extract code in its own module Given about 2/3 or 'mercurial.repair' is now about repository upgrade, I think it is fair to move it into its own module. An expected benefit is the ability to drop the 'upgrade' prefix of many functions. This will be done in coming changesets.

bundle2: move the 'close' method off the unpackermixin This method is unrelated to unpacking and only used by the unbundle20 class. We move it there for clarity.

bundle2: clarify the docstring of unpackermixin methods The unpackermixin is a utility used to implement the bundle2 protocol. It should not be used when writing part handlers. We update the docstring to clarify this.