Implement pex3 lock sync. (#2373)

Introduce the high level `pex3 lock sync` command. This should generally
suffice for typical use cases and works as follows:

+ On first use (where the specified `--lock` does not yet exist), it
  acts just like `pex3 lock create`.
+ On subsequent uses it does a minimal synchronization of the lock based
  on the diff of the given current requirements against the requirements
  used to generate the specified `--lock`. This amounts to formulating a
  `pex3 lock update` command with the appropriate `-p`, `-R` and `-d`
  arguments.

In addition to creating and syncing a lock, it can also create and sync
a venv (--venv) based on the lock. Further, a command can be specified
to run in the synchronized venv with arguments following the `--` option
terminator.

This latter set of features allow Pex to act as a concise tool in `tox`
/
`nox` / `invoke` / `make` setups to implement a simple build system.

Fixes #2344

---------

Co-authored-by: Andreas Stenius <[email protected]>

Author: jsirois

pex/cli/command.py

@@ -8,7 +8,7 @@
 from contextlib import contextmanager
 
 from pex.commands.command import Command
-from pex.result import Result
+from pex.result import Error, Result
 from pex.typing import TYPE_CHECKING, Generic, cast
 
 if TYPE_CHECKING:
@@ -35,11 +35,23 @@ def parser(
             help,  # type: str
             func=None,  # type: Optional[Callable[[_C], Result]]
             include_verbosity=None,  # type: Optional[bool]
+            passthrough_args=None,  # type: Optional[str]
         ):
             # type: (...) -> Iterator[ArgumentParser]
             subcommand_parser = self._subparsers.add_parser(name=name, help=help)
             yield subcommand_parser
             if func:
+                if passthrough_args:
+                    # N.B.: This is a dummy arg for usage string display purposes; thus the "_" name.
+                    subcommand_parser.add_argument(
+                        "_", metavar="-- passthrough args", nargs="*", help=passthrough_args
+                    )
+                else:
+                    func = functools.partial(
+                        BuildTimeCommand._check_no_passthrough_args_and_run,
+                        subcommand_name=name,
+                        subcommand_func=func,
+                    )
                 subcommand_parser.set_defaults(subcommand_func=func)
                 Command.register_global_arguments(
                     subcommand_parser,
@@ -75,6 +87,20 @@ def create_subcommands(
         subparsers = parser.add_subparsers(description=description)
         return cls.Subcommands(subparsers, include_verbosity=cls.include_global_verbosity_option)
 
+    def _check_no_passthrough_args_and_run(
+        self,
+        subcommand_name,  # type: str
+        subcommand_func,  # type: Callable[[BuildTimeCommand], Result]
+    ):
+        # type: (...) -> Result
+        if self.passthrough_args is not None:
+            return Error(
+                "The {subcommand} {command} subcommand does not accept pass through args.".format(
+                    subcommand=subcommand_name, command=self.name()
+                )
+            )
+        return subcommand_func(self)
+
     def run(self):
         # type: (_C) -> Result
         subcommand_func = cast(