Handle newly added transitive dependencies when updating transitive dependencies

[Stebalien]

If a dependency is mising from the dependency tree when starting a gx-workspace update start, transitive dependencies will not be updated. By example...

Given the following dependency tree:

a(1)
+- b(1)
   +- c(1)
      +- d(1)

Update b to version 2, adding a new dependency, x, with a transitive dependency on d. DO NOT propagate this to update a:

b(2)
|- c(1)
|  +- d(1)
|
+- x(1)
    +- d(1)

Separately, update d to version 2 and attempt to propegate this update to a (run gx-workspace update ... on a).

We should get:

a(1)
+- b(3)
   +- c(2)
   |  +- d(2)
   |
   +- x(2)
       +- d(2)

We actually get:

a(1)
+- b(3)
   +- c(2)
   |  +- d(2)
   |
   +- x(1)
       +- d(1) // not updated.