pip: Allow overriding system_packages list with ignore-installed arg

`flatpak-pip-generator --ignore-installed pip pip` will now generate
an actual manifest and append `--ignore-installed` to the build
command

Fixes #254

Author: bbhtt

pip/flatpak-pip-generator

@@ -377,7 +377,7 @@ for package in packages:
         print('Warning: skipping invalid requirement specification {} because it is missing a name'.format(package.line), file=sys.stderr)
         print('Append #egg=<pkgname> to the end of the requirement line to fix', file=sys.stderr)
         continue
-    elif not opts.python2 and package.name.casefold() in system_packages:
+    elif not opts.python2 and package.name.casefold() in system_packages and not package.name.casefold() in opts.ignore_installed:
         print(f"{package.name} is in system_packages. Skipping.")
         continue
 
@@ -413,7 +413,7 @@ for package in packages:
             subprocess.run(pip_download + [pkg], check=True, stdout=subprocess.DEVNULL)
             for filename in sorted(os.listdir(tempdir)):
                 dep_name = get_package_name(filename)
-                if dep_name.casefold() in system_packages:
+                if dep_name.casefold() in system_packages and not dep_name.casefold() in opts.ignore_installed:
                     continue
                 dependencies.append(dep_name)