feat(graph): Improve why command output

[dhellmann]

The graph why command is a tool for developers to understand the
dependency graph. This change improves its output to be more useful and
clearer.

The output now includes the requirement specifier from the parent package
along with the parent package name. This makes it easier to understand
not just which package is pulling in a dependency, but which version or
range of versions it requires.

The output for top-level dependencies specified on the command line or
in a requirements file is also improved to clearly label them as such.

Cycle detection is included to prevent infinite loops and make it clear
where circular dependencies exist.

[rd4398]

Looks good to me!

[dhellmann]

I should have included some sample output:

$ fromager graph why --depth 5 ./e2e/graph-with-dependency-conflict.json docling-parse

docling-parse==4.0.0
 * docling-parse==4.0.0 is a toplevel dependency with req docling-parse==4.0.0
 * docling-parse==4.0.0 is an install dependency of docling==2.28.4 with req docling-parse<5.0.0,>=4.0.0
   * docling==2.28.4 is a toplevel dependency with req docling==2.28.4

docling-parse==2.1.2
 * docling-parse==2.1.2 is an install dependency of instructlab-sdg==0.7.2 with req docling-parse<3.0.0,>=2.0.0
   * instructlab-sdg==0.7.2 is a toplevel dependency with req instructlab-sdg==0.7.2
   * instructlab-sdg==0.7.2 is an install dependency of instructlab==0.23.5 with req instructlab-sdg<0.8.0,>=0.7.2
     * instructlab==0.23.5 is a toplevel dependency with req instructlab[cuda]==0.23.5
 * docling-parse==2.1.2 is an install dependency of docling==2.8.3 with req docling-parse<3.0.0,>=2.0.5
   * docling==2.8.3 is an install dependency of instructlab-sdg==0.7.2 with req docling[tesserocr]<=2.8.3,>=2.4.2; sys_platform != "darwin"
     * instructlab-sdg==0.7.2 is a toplevel dependency with req instructlab-sdg==0.7.2
     * instructlab-sdg==0.7.2 is an install dependency of instructlab==0.23.5 with req instructlab-sdg<0.8.0,>=0.7.2
       * instructlab==0.23.5 is a toplevel dependency with req instructlab[cuda]==0.23.5