Package derivations

[dread]

A 'Derived' relationship can be applied from one package to another.

e.g. sussex-demography is derived from census-2001

'Derived' relationship is:

• directional
• many:many
• stateful

'derived' table columns:

• id (primary key)
• source_package (foreign key)
• result_package (foreign key)
• description (markdown text)

Further tickets:

• WUI - package view - shows 'derives from package x' and 'derived package y' with UML-like diagram of x -> this package -> y
• WUI - package edit form - new option to say it 'derives from' or 'has derivation' and you select the appropriate
• REST if - expose reading and writing this property

[dread]

Covered in ticket:253