Allow vehicles to not start at the depot; real-time passenger pick up and drop-off

At the moment all routes start and end at a single depot. Always.
We should allow for vehicles to start already "on the road".

Use-case: real-time passenger pick up and drop-off "ride sharing".

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Implementation:

Add dummy depot with artificial edges to all start locations. Set edge costs to zero.

No files, tests, or entry points are named. Trace the route and depot construction and locate the existing routing tests first; the work is done when a dummy depot can connect to all start locations with zero-cost edges while existing depot-based routes continue to work.

Written by the indexing model from the issue text.