feat: implement DependencyGraph.get_build_topology

[tiran]

The DependencyGraph.get_build_topology method returns a TrackingTopologicalSorter with all nodes in the graph. Each node tracks its build dependency set. Nodes for a package with exclusive builds are flagged as exclusive.

The new fromager graph build-graph subcommand analyzes a graph file and prints a list of build steps.

$ fromager graph build-graph e2e/build-parallel/graph.json 
Build dependencies (6):
cython==3.1.1, flit-core==3.12.0, packaging==25.0, setuptools-scm==8.3.1, setuptools==80.8.0, wheel==0.46.1 

Build rounds:
1. flit-core==3.12.0, setuptools==80.8.0
2. cython==3.1.1, imapclient==3.0.1, jinja2==3.1.6, markupsafe==3.0.2, more-itertools==10.7.0, packaging==25.0
3. setuptools-scm==8.3.1, wheel==0.46.1
4. imapautofiler==1.14.0, jaraco-classes==3.4.0, jaraco-context==6.0.1, jaraco-functools==4.1.0, keyring==25.6.0, pyyaml==6.0.2

Building 16 packages in 4 rounds.

[tiran]

@LalatenduMohanty This PR is the next step after #795

[LalatenduMohanty]

@tiran thanks, I am assuming #796 needs it as well.

[tiran]

@tiran thanks, I am assuming #796 needs it as well.

I'm taking 796 apart into smaller chunks to make the code easier to review and land.

[rd4398]

This looks good! I have left a small comment but it is non-blocking for this MR

[rd4398]

This looks good!