Make mkylib reusable and install it

[jktjkt]

I'm looking at using yangson with @Telecominfraproject's oopt-gnpy because yangson looks like something that can validate JSON files against a YANG schema. Thanks for the tool!

One thing which made the workflow more complex for me was the fact that mkylib was not installed via pip install yangson. This patch therefore makes it available as yangson-mkylib. While at it, I also adjusted the wrapping a bit so that the code is usable from Python without direct sys.exit calls, print(), etc.:

>>> import json
>>> from yangson.mkylib import process_directory as PPP
>>> print(json.dumps(PPP("yang/"), indent=4))
{
    "ietf-yang-library:modules-state": {
        "module-set-id": "",
        "module": [
            {
                "name": "ietf-network-topology",
                "revision": "2018-02-26",
                "namespace": "urn:ietf:params:xml:ns:yang:ietf-network-topology",
                "conformance-type": "implement"
            },
            {
                "name": "ietf-network",
                "revision": "2018-02-26",
                "namespace": "urn:ietf:params:xml:ns:yang:ietf-network",
                "conformance-type": "implement"
            },
            {
                "name": "ietf-yang-types",
                "revision": "2013-07-15",
                "namespace": "urn:ietf:params:xml:ns:yang:ietf-yang-types",
                "conformance-type": "import"
            },
            {
                "name": "ietf-geo-location",
                "revision": "2019-02-17",
                "namespace": "urn:ietf:params:xml:ns:yang:ietf-geo-location",
                "feature": [
                    "alternate-systems"
                ],
                "conformance-type": "import"
            },
            {
                "name": "tip-photonic-topology",
                "revision": "2019-07-02",
                "namespace": "https://oopt.telecominfraproject.com/yang/topology",
                "conformance-type": "implement"
            },
            {
                "name": "ietf-inet-types",
                "revision": "2013-07-15",
                "namespace": "urn:ietf:params:xml:ns:yang:ietf-inet-types",
                "conformance-type": "import"
            },
            {
                "name": "tip-photonic-equipment",
                "revision": "2019-06-20",
                "namespace": "https://oopt.telecominfraproject.com/yang/equipment",
                "conformance-type": "implement"
            }
        ]
    }
}
>>>

[llhotka]

Ahoj @jktjkt, I am sorry for responding so late. Regarding your PR, I am somewhat concerned that by making it an “official” script we could mislead users into thinking that it solves the problem of creating a YANG library file completely. It is usually not the case – modules from other directories often have to be added.