Have --license work with --reverse

[kemzeb]

In #318 I did not consider the case where users would pass --reverse along with --license. Currently, doing so would have us output the following:

$ pipdeptree --reverse --license -d 1
chardet==5.2.0
└── diff-cover==8.0.3 [requires: chardet>=3.0.0] (Apache Software License)
coverage==7.4.2
├── covdefaults==2.3.0 [requires: coverage>=6.0.2] (MIT License)
└── pytest-cov==4.1.0 [requires: coverage>=5.2.1] (MIT License)
distlib==0.3.8
└── virtualenv==20.25.1 [requires: distlib>=0.3.7,<1] (MIT License)
exceptiongroup==1.2.0
└── pytest==8.0.1 [requires: exceptiongroup>=1.0.0rc8] (MIT License)
filelock==3.13.1
└── virtualenv==20.25.1 [requires: filelock>=3.12.2,<4] (MIT License)
iniconfig==2.0.0
└── pytest==8.0.1 [requires: iniconfig] (MIT License)
MarkupSafe==2.1.5
└── Jinja2==3.1.3 [requires: MarkupSafe>=2.0] (BSD License)
packaging==23.2
└── pytest==8.0.1 [requires: packaging] (MIT License)
pip==24.0
pipdeptree==0.1.dev418+gc6de3c3
. . . .

As it's shown, the required packages (i.e. graph nodes that were instances of DistPackage) are given their license when the top-level packages should have been given theirs instead. This PR addresses this text render issue by appending the license to a package only if it's a top-level package (instead of just checking to see if node is an instance of DistPackage).

The output with my changes:

$ pipdeptree --reverse --license -d 1
chardet==5.2.0 (GNU Lesser General Public License v2 or later (LGPLv2+))
└── diff-cover==8.0.3 [requires: chardet>=3.0.0]
coverage==7.4.2 (Apache Software License)
├── covdefaults==2.3.0 [requires: coverage>=6.0.2]
└── pytest-cov==4.1.0 [requires: coverage>=5.2.1]
distlib==0.3.8 (Python Software Foundation License)
└── virtualenv==20.25.1 [requires: distlib>=0.3.7,<1]
exceptiongroup==1.2.0 (MIT License)
└── pytest==8.0.1 [requires: exceptiongroup>=1.0.0rc8]
filelock==3.13.1 (The Unlicense (Unlicense))
└── virtualenv==20.25.1 [requires: filelock>=3.12.2,<4]
iniconfig==2.0.0 (MIT License)
└── pytest==8.0.1 [requires: iniconfig]
MarkupSafe==2.1.5 (BSD License)
└── Jinja2==3.1.3 [requires: MarkupSafe>=2.0]
packaging==23.2 (Apache Software License, BSD License)
└── pytest==8.0.1 [requires: packaging]
pip==24.0 (MIT License)
pipdeptree==0.1.dev418+gc6de3c3 (MIT License)
. . . .

[gaborbernat]