container: enter init path early

There is generally no use case where any setup is required before init, and requiring the explicit function call is error-prone and unnecessary. It also causes trouble with packages using a similar trick. This change moves argv0 check early and makes it an import side effect. The stub will be removed in v0.5.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/pkg_test.go

@@ -85,8 +85,6 @@ func newRContext(tb testing.TB, c *pkg.Cache) *pkg.RContext {
 	return &r
 }
 
-func TestMain(m *testing.M) { container.TryArgv0(nil); os.Exit(m.Run()) }
-
 // overrideIdent overrides the ID method of [Artifact].
 type overrideIdent struct {
 	id pkg.ID