This is accomplished with the :reduce metadata, which specifies the
reduce function to use when merging. This allows us to merge
dependencies and repositories deeply like other structures. Note that
dependencies are transformed into a map before they are merged and then
transformed back into a vector.
Also change the way that collections are merged. They used to be merged
by taking the right collection and prepending it to the left collection.
This behavior was needed for :*-paths in defproject, but it is not an
obvious default. Now, the default is to append the right collection, but
the :prepend metadata can be used to tell meta-merge to prepend instead.
By default, :source-paths, :resource-paths and :test-paths have :prepend
set to true.
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