paket simplify does not operate on groups in paket.references

Description

The paket simplify function will reduce this paket.references:

Castle.Core
Castle.Windsor

to this

Castle.Windsor

However, if the references are in a group it will have no effect, e.g.:

group Foo
Castle.Core
Castle.Windsor

remains

group Foo
Castle.Core
Castle.Windsor

though paket.dependencies is simplified correctly.

Repro steps

1. Create a new project

2. Add Castle.Windsor via nuget.

3. Run paket convert-from-nuget

4. Alter paket.dependencies and paket.references to put the packages in a group (i.e. add "group Foo" to the top of each file and indent the remaining contents).

5. Run paket install

6. Run paket simplify

Expected behavior

paket.dependencies will be simplified (the reference to Castle.Core will be removed).
paket.references will be simplified (the reference to Castle.Core will be removed).

Actual behavior

paket.dependencies will be simplified (the reference to Castle.Core will be removed).
paket.references will be unchanged.

Known workarounds

None, if you have multiple groups.

Start by reproducing the issue with grouped paket.dependencies and paket.references, then trace the paket simplify handling for paket.references and compare it with the working paket.dependencies behavior. Done means the grouped paket.references removes Castle.Core while preserving the group; verify with paket install followed by paket simplify.