Optional fullstack feature is not activated when passing the `--fullstack` flag

**Problem**

If your package is set up with a feature to enable `dioxus/fullstack` the new version of dx doesn't enable that feature even if you pass the `--fullstack` flag

**Steps To Reproduce**

Steps to reproduce the behavior:

- clone this repo: https://github.com/ealmloff/dx-optional-fullstack with this feature setup:
```toml
[dependencies]
dioxus = { version = "0.7.0-rc.1", features = ["router"] }

[features]
default = ["web"]
web = ["dioxus/web"]
fullstack = ["dioxus/fullstack"]
server = ["dioxus/server"]
```

- run `dx serve --fullstack`
- observe the fullstack feature is not enabled and hydration fails because the client wasn't built with hydraiton enabled

**Environment:**

- Dioxus version: 0.7.0-rc.1
- Rust version: nightly
- OS info: macOS
- App platform: web + fullstack