Import the os-release parser from the man page

Fixes https://pagure.io/fedora-rust/rust2rpm/issue/159.

The code in the man page is licensed as CC-0, so it's fine to just
copy it here. (And I wrote it, so it would be fine anyway ;)).

rust2rpm/__main__.py

@@ -1,4 +1,5 @@
 import argparse
+import ast
 import configparser
 import contextlib
 from datetime import datetime, timezone
@@ -52,26 +53,33 @@ def sortify(func):
         return sorted(func(*args, **kwargs))
     return functools.update_wrapper(wrapper, func)
 
-def get_default_target():
+def read_os_release():
     try:
-        os_release_file = open('/etc/os-release')
+        f = open('/etc/os-release')
     except FileNotFoundError:
-        os_release_file = open('/usr/lib/os-release')
-    with os_release_file:
-        conf = configparser.ConfigParser()
-        conf.read_file(itertools.chain(["[os-release]"], os_release_file))
-        os_release = conf["os-release"]
+        f = open('/usr/lib/os-release')
+
+    for line in f:
+        line = line.rstrip()
+        if not line or line.startswith('#'):
+            continue
+        m = re.match(r'([A-Z][A-Z_0-9]+)=(.*)', line)
+        if m:
+            name, val = m.groups()
+            if val and val[0] in '"\'':
+                val = ast.literal_eval(val)
+            yield name, val
+
+def get_default_target():
+    os_release = dict(read_os_release())
     os_id = os_release.get("ID")
-    os_like = os_release.get("ID_LIKE")
-    if os_like is not None:
-        os_like = shlex.split(os_like)
-    else:
-        os_like = []
+    # ID_LIKE is a space-separated list of identifiers like ID
+    os_like = os_release.get("ID_LIKE", "").split()
 
     # Order matters here!
-    if os_id == "mageia" or ("mageia" in os_like):
+    if "mageia" in (os_id, *os_like):
         return "mageia"
-    elif os_id == "fedora" or ("fedora" in os_like):
+    elif "fedora" in (os_id, *os_like):
         return "fedora"
     elif "suse" in os_like:
         return "opensuse"