set up new mechanism for unlisted subcommands

For the build automation, there will be lots of subcommands that are meant
to run in scripted environments.  They are not intended for interactive use.
Although they sometimes might be useful interactively, they might also
sometimes be dangerous to run interactively because they rely on the
presense of a VM/container, modify the local environment, or even run
things as root.

fdroidserver/__main__.py

@@ -55,6 +55,18 @@ COMMANDS = OrderedDict([
     ("mirror", _("Download complete mirrors of small repos")),
 ])
 
+# The list of subcommands that are not advertised as public api,
+# intended for the use-case of breaking down builds into atomic steps.
+#
+# For the build automation, there will be lots of subcommands that are
+# meant to run in scripted environments.  They are not intended for
+# interactive use.  Although they sometimes might be useful
+# interactively, they might also sometimes be dangerous to run
+# interactively because they rely on the presense of a VM/container,
+# modify the local environment, or even run things as root.
+COMMANDS_INTERNAL = [
+]
+
 
 def print_help(available_plugins=None):
     print(_("usage: ") + _("fdroid [<command>] [-h|--help|--version|<args>]"))
@@ -136,7 +148,12 @@ def main():
         sys.exit(0)
 
     command = sys.argv[1]
-    if command not in COMMANDS and command not in available_plugins:
+    command_not_found = (
+        command not in COMMANDS
+        and command not in COMMANDS_INTERNAL
+        and command not in available_plugins
+    )
+    if command_not_found:
         if command in ('-h', '--help'):
             print_help(available_plugins=available_plugins)
             sys.exit(0)
@@ -186,7 +203,7 @@ def main():
     sys.argv[0] += ' ' + command
 
     del sys.argv[1]
-    if command in COMMANDS.keys():
+    if command in COMMANDS.keys() or command in COMMANDS_INTERNAL:
         # import is named import_subcommand internally b/c import is reserved by Python
         command = 'import_subcommand' if command == 'import' else command
         mod = __import__('fdroidserver.' + command, None, None, [command])

fdroidserver/common.py

@@ -1046,6 +1046,27 @@ def get_local_metadata_files():
     return glob.glob('.fdroid.[a-jl-z]*[a-rt-z]')
 
 
+def split_pkg_arg(appid_versionCode_pair):
+    """Split 'appid:versionCode' pair into 2 separate values safely.
+
+    :raises ValueError: if argument is not parseable
+    :return: (appid, versionCode) tuple with the 2 parsed values
+    """
+    tokens = appid_versionCode_pair.split(":")
+    if len(tokens) != 2:
+        raise ValueError(
+            _("'{}' is not a valid pair of the form appId:versionCode pair").format(
+                appid_versionCode_pair
+            )
+        )
+    if not is_valid_package_name(tokens[0]):
+        raise ValueError(
+            _("'{}' does not start with a valid appId").format(appid_versionCode_pair)
+        )
+    versionCode = version_code_string_to_int(tokens[1])
+    return tokens[0], versionCode
+
+
 def read_pkg_args(appid_versionCode_pairs, allow_version_codes=False):
     """No summary.