add a graph why command to explain why certain package appears in the graph

[shubhbapna]

fixes #462

Didn't get the bonus marks for converting to dot file : (

[shubhbapna]

Here are some examples:

$ fromager graph why ./graph.json dill

dill==0.3.8
   * is an install dependency of datasets==3.0.1 with req dill<0.3.9,>=0.3.0
   * is an install dependency of datasets==2.21.0 with req dill<0.3.9,>=0.3.0

dill==0.3.9
   * is an install dependency of multiprocess==0.70.16 with req dill>=0.3.8
   * is an install dependency of lm-eval==0.4.4 with req dill
   * is an install dependency of evaluate==0.4.3 with req dill

$ fromager graph why ./graph.json dill --version 0.3.9

dill==0.3.9
   * is an install dependency of multiprocess==0.70.16 with req dill>=0.3.8
   * is an install dependency of lm-eval==0.4.4 with req dill
   * is an install dependency of evaluate==0.4.3 with req dill

$ fromager graph why ./graph.json dill --recursive-depth 1

dill==0.3.8
   * is an install dependency of datasets==3.0.1 with req dill<0.3.9,>=0.3.0
     * is an install dependency of instructlab==0.19.0 with req datasets>=2.18.0
     * is an install dependency of trl==0.11.1 with req datasets
     * is an install dependency of instructlab-training==0.5.0 with req datasets>=2.15.0
     * is an install dependency of instructlab-training==0.5.0 with req datasets>=2.15.0
     * is an install dependency of lm-eval==0.4.4 with req datasets>=2.16.0
     * is an install dependency of evaluate==0.4.3 with req datasets>=2.0.0
     * is an install dependency of outlines==0.0.46 with req datasets
   * is an install dependency of datasets==2.21.0 with req dill<0.3.9,>=0.3.0
     * is an install dependency of instructlab-sdg==0.3.0 with req datasets<3.0.0,>=2.18.0

dill==0.3.9
   * is an install dependency of multiprocess==0.70.16 with req dill>=0.3.8
     * is an install dependency of datasets==3.0.1 with req multiprocess
     * is an install dependency of datasets==2.21.0 with req multiprocess
     * is an install dependency of evaluate==0.4.3 with req multiprocess
   * is an install dependency of lm-eval==0.4.4 with req dill
     * is an install dependency of instructlab-eval==0.3.1 with req lm-eval>=0.4.4
   * is an install dependency of evaluate==0.4.3 with req dill
     * is an install dependency of lm-eval==0.4.4 with req evaluate
     * is an install dependency of lm-eval==0.4.4 with req evaluate>=0.4.0

$ fromager graph why ./graph.json numpy --req-type build

numpy==1.26.4
   * is an build-system dependency of torch==2.3.1 with req numpy<2.0.0
   * is an build-system dependency of pandas==2.2.1 with req numpy<=2.0.0.dev0,>1.22.4
   * is an build-system dependency of vllm==0.5.2.3 with req numpy<2.0.0
   * is an build-system dependency of xformers==0.0.27 with req numpy<2.0.0

numpy==2.1.1
   * is an build-system dependency of pyarrow==17.0.0 with req numpy>=1.25; python_version >= "3.9"
   * is an build-system dependency of scikit-learn==1.5.2 with req numpy>=2
   * is an build-system dependency of scipy==1.14.1 with req numpy<2.3,>=2.0.0rc1
   * is an build-system dependency of numexpr==2.10.1 with req numpy>=2.0.0

numpy==2.0.2
   * is an build-backend dependency of numba==0.60.0 with req numpy<2.1,>=2.0.0rc1
   * is an build-sdist dependency of numba==0.60.0 with req numpy<2.1,>=2.0.0rc1