Add Enumerable#group_by
Merge repeated deps with differing options

When expanding dependencies, repeated deps are treated as equal and all
but the first are discarded when #uniq is called on the resulting array.
However, they may have different sets of options attached, so we cannot
assume they are the same.

After the initial expansion, we group them by name and then create a new
Dependency object for each name, merging the options from each group.

Fixes #20335.