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.

Cargo.toml

@@ -8,6 +8,10 @@ members = [
 
     "ports/cef",
 ]
+default-members = [
+    "ports/servo",
+    "tests/unit/*",
+]
 exclude = [".cargo"]
 
 [profile.dev]

python/servo/build_commands.py

@@ -402,8 +402,7 @@ class MachCommands(CommandBase):
         self.ensure_clobbered()
 
         ret = None
-        opts = []
+        opts = ["-p", "embedding"]
-        opts += ["--manifest-path", self.cef_manifest()]
 
         if jobs is not None:
             opts += ["-j", jobs]
@@ -458,8 +457,7 @@ class MachCommands(CommandBase):
         env = self.build_env(is_build=True, geckolib=True)
 
         ret = None
-        opts = []
+        opts = ["-p", "geckoservo"]
-        opts += ["--manifest-path", self.geckolib_manifest()]
         features = []
 
         if jobs is not None:

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',