Major cleanup (new library loader, rm multilib, rm stdcall)

This removes support for multiple different libraries in one file, which
IOHO was an inherent design mistake messing up the code - pypdfium2
ctypesgen is now single-lib.

We also replace the old, bloated library loader with a new lean one.
In the future, we may want an option to plug in a custom library loader
so we can import config from pypdfium2.

This also removes support for the windows-only stdcall convention
because it's not needed for pypdfium2 and probably an edge case.
It would cause some additional complexity and we're not sure what would
be the cleanest way to integrate it, so this can wait.

Fun fact:
It seems like ctypesgen prior to this change added the complie_libdirs
to the runtime library loader. Further, relative paths (I think) were
never correctly interpreted relative to the file's directory, but to the
user's CWD, which is useless and could even cause unexpected behavior.
So runtime_libdirs = ["."] didn't work, but the fallback below "then we
search the directory where the generated python interface is stored"
captured.

ctypesgen/__main__.py

@@ -45,20 +45,20 @@ def main(givenargs=None):
         nargs="+",
         help="Sequence of header files",
     )
-    parser.add_argument(
-        "-o",
-        "--output",
-        metavar="FILE",
-        help="write wrapper to FILE [default stdout]",
-    )
     parser.add_argument(
         "-l",
-        "--libraries",
-        nargs="+",
+        "--library",
+        required=True,
         default=[],
         metavar="LIBRARY",
         help="link to LIBRARY",
     )
+    parser.add_argument(
+        "-o",
+        "--output",
+        metavar="FILE",
+        help="write wrapper to FILE [default stdout]",
+    )
     parser.add_argument(
         "--other-headers",
         nargs="+",
@@ -322,8 +322,7 @@ def main(givenargs=None):
     # Figure out what names will be defined by imported Python modules
     args.other_known_names = find_names_in_modules(args.modules)
 
-    if len(args.libraries) == 0:
-        msgs.warning_message("No libraries specified", cls="usage")
+    assert args.library
 
     # Fetch printer for the requested output language
     if args.output_language == "py":