SDK Manager: XML Schema for sdk-repository v2

SDK Manager: XML Schema for sdk-repository v2

v2 will be a full standalone schema, not reusing any parts of v1.
Each package type is now defined using a named type, which means
a v3 version would redifine the top element and could include
some named types of v2.

Unfortunately we cannot *extend* the top element from v1 because
extension in XML Schema seems to imply a sequence (i.e. fixed order)
whereas we use a "choice" mode to have 0..N of each types in any
order.

SDK BUG 2252825

Change-Id: I6809bd4486ef2d7f2473995ed5a0abb7fa377bdd