Let's talk about dependencies:
Project Repository is needed by:
-- Project Scheduling
-- Project based todo list
-- Resource Allocation Management
-- Timekeeping
-- Expenses (at least in most cases it makes sense to link them to a project)
Scheduling is needed by:
-- Project Scheduling
-- Task / Todo List (for keeping track of due dates)
Resource Management is needed by:
-- Time Keeping
-- Holiday Booking
-- Expenses
-- Todo List
If you agree with this aproach, we could have a closer look what we need, which tasks need to be done first and then find the people to work on the tasks in a given order. My assumption is, once we have the project repository and the resource management ready in the basics, other packages could be developed in parallel.