fix: make topological sorting support fully cyclic dependencies

[schmelczer]

In case the graph of dependencies only consisted of a directed cycle, sort_topologically returned an empty list. This was an issue when trying to sort the dependencies of an environment containing Python 3.12 and Pip on MacOS as both depend on each other. See the relevant section of the repodata from conda forge:

[{
    "build": "h1411813_0_cpython",
    "build_number": 0,
    "constrains": [
        "python_abi 3.12.* *_cp312"
    ],
    "depends": [
        "__osx >=10.9",
        "bzip2 >=1.0.8,<2.0a0",
        "libexpat >=2.6.2,<3.0a0",
        "libffi >=3.4,<4.0a0",
        "libsqlite >=3.45.2,<4.0a0",
        "libzlib >=1.2.13,<1.3.0a0",
        "ncurses >=6.4.20240210,<7.0a0",
        "openssl >=3.2.1,<4.0a0",
        "readline >=8.2,<9.0a0",
        "tk >=8.6.13,<8.7.0a0",
        "tzdata",
        "xz >=5.2.6,<6.0a0",
        "pip"
    ],
    "license": "Python-2.0",
    "md5": "df1448ec6cbf8eceb03d29003cf72ae6",
    "name": "python",
    "sha256": "3b327ffc152a245011011d1d730781577a8274fde1cf6243f073749ead8f1c2a",
    "size": 14557341,
    "subdir": "osx-64",
    "timestamp": 1713208068012,
    "version": "3.12.3",
    "fn": "python-3.12.3-h1411813_0_cpython.conda",
    "url": "https://conda.anaconda.org/conda-forge/osx-64/python-3.12.3-h1411813_0_cpython.conda",
    "channel": "https://conda.anaconda.org/conda-forge/"
},
{
    "build": "pyhd8ed1ab_0",
    "build_number": 0,
    "depends": [
        "python >=3.7",
        "setuptools",
        "wheel"
    ],
    "license": "MIT",
    "license_family": "MIT",
    "md5": "f586ac1e56c8638b64f9c8122a7b8a67",
    "name": "pip",
    "noarch": "python",
    "sha256": "b7c1c5d8f13e8cb491c4bd1d0d1896a4cf80fc47de01059ad77509112b664a4a",
    "size": 1398245,
    "subdir": "noarch",
    "timestamp": 1706960660581,
    "version": "24.0",
    "fn": "pip-24.0-pyhd8ed1ab_0.conda",
    "url": "https://conda.anaconda.org/conda-forge/noarch/pip-24.0-pyhd8ed1ab_0.conda",
    "channel": "https://conda.anaconda.org/conda-forge/"
},
...
]

Even though topological sorting theoretically requires a DAG as its input, the existing implementation of sort_topologically already handled directed cycles just not the case where all the nodes were part of such cycles.

The fix is that if we can't find any root nodes, we start cycle detection from a random (the first in the given list) node.

[baszalmstra]

Thanks!