Use workspace.default-members to specify default crates for 'cargo build'

… and 'cargo test', etc. Include Servo and its unit tests,
but not Stylo because that would try to compile the style
crate with incompatible feature flags:
https://github.com/rust-lang/cargo/issues/4463

`workspace.default-members` was added in
https://github.com/rust-lang/cargo/pull/4743.
Older Cargo versions ignore it.

python/servo/devenv_commands.py

@@ -94,7 +94,7 @@ class MachCommands(CommandBase):
         'params', default=None, nargs='...',
         help="Command-line arguments to be passed through to cargo check")
     def check_geckolib(self, params):
-        return self.run_cargo(params, check=True, geckolib=True)
+        return self.run_cargo(["-p", "geckoservo"] + (params or []), check=True, geckolib=True)
 
     @Command('cargo-update',
              description='Same as update-cargo',