scripts/pythondistdeps: Backport switch to importlib.metadata from upstream

Upstream change to importlib.metadata: https://github.com/rpm-software-management/rpm/pull/1317

Due to extras packages being hadled slightly differently by importlib,
one test case for this was added.  And due to changes in handling
requires.txt files, comments were removed from the pyreq2rpm.tests
testing package.

Also because of the switch, we removed the dependency on setuptools and
added a dependency on packaging.

Note: Some packages with egg-info files might provide a different name
due to this change if there is a conflict between the filename and the
name in the metadata. Previously, the filename was sometimes used to
parse the name, now it is always the content of that file, which is what
packaging does, and thus also pip and other Python tooling. Currently,
this is known to affect only 1 package in Fedora (ntpsec).

The resulting script is different from upstream because of not yet upstreamed changes in Fedora:
- scripts/pythondistdeps: Rework error messages
- scripts/pythondistdeps: Add parameter --package-name
- scripts/pythondistdeps: Implement provides/requires for extras packages
- pythondistdeps.py: When parsing extras name, take the rightmost +

These changes are proposed in this upstream PR: https://github.com/rpm-software-management/rpm/pull/1546

tests/data/scripts_pythondistdeps/pyreq2rpm.tests-2020.04.07.024dab0-py3.9.egg-info/requires.txt

@@ -1,4 +1,3 @@
-# Taken from pyreq2rpm, removed tests that are expected to fail
 foobar0~=2.4.8
 foobar1~=2.4.8.0
 foobar2~=2.4.8.1
@@ -91,10 +90,8 @@ pyparsing0
 pyparsing1>=2.0.1,!=2.0.4,!=2.1.2,!=2.1.6
 babel>=1.3,!=2.0
 
-# Tests for breakages in Fedora
 fedora-python-nb2plots==0+unknown
 
-# Other tests
 hugo1==1.0.0.dev7
 hugo2<=8a4
 hugo3!=11.1.1b14

tests/data/scripts_pythondistdeps/test-data.yaml

@@ -1216,6 +1216,15 @@
                 python3dist(backports-range) = 3.7.2
                 python3dist(backports.range) = 3.7.2
             requires: python(abi) = 3.7
+--requires --normalized-names-format pep503 --package-name python3-setuptools+certs:
+    --provides --majorver-provides --normalized-names-format pep503 --package-name python3-setuptools+certs:
+        usr/lib/python3.9/site-packages/setuptools-41.6.0.dist-info:
+            provides: |-
+                python3.9dist(setuptools[certs]) = 41.6
+                python3dist(setuptools[certs]) = 41.6
+            requires: |-
+                python(abi) = 3.9
+                python3.9dist(certifi) = 2016.9.26
 --requires --normalized-names-format pep503 --package-name python3-zope-component+testing:
     --provides --majorver-provides --normalized-names-format pep503 --package-name python3-zope-component+testing:
         usr/lib/python3.9/site-packages/zope.component-4.3.0-py3.9.egg-info: