message: Rework namespace formatting
diff: @@ -205,7 +205,7 @@ impl Init {
  // generate the Wapm struct for the [metadata.wapm] table
  // and add it to the end of the file
  let metadata_wapm = wapm_toml::rust::Wapm {
  - namespace: constructed_manifest.namespace.clone(),
  + namespace: constructed_manifest.namespace.as_deref().unwrap_or("_").to_string(),
  package: Some(constructed_manifest.module_name.clone()),
  wasmer_extra_flags: None,
  abi: constructed_manifest.default_abi,
  @@ -395,7 +395,7 @@ impl Init {
  }
  struct ConstructManifestReturn {
  - namespace: String,
  + namespace: Option<String>,
  default_abi: wapm_toml::Abi,
  module_name: String,
  bindings: Option<wapm_toml::Bindings>,
  @@ -415,11 +415,10 @@ fn construct_manifest(
  ) -> ConstructManifestReturn {
  let package_name = cargo_toml.as_ref().map(|p| &p.name).unwrap_or(package_name);
  - let namespace = namespace.clone().unwrap_or_else(|| {
  + let namespace = namespace.clone().or_else(|| {
  let username = wasmer_registry::whoami(None, None).ok().map(|o| o.1);
  username
  .or_else(|| package_name.split('/').next().map(|s| s.to_string()))
  - .unwrap_or_else(|| "_".to_string())
  });
  let module_name = package_name
  .split('/')
  @@ -482,7 +481,7 @@ fn construct_manifest(
  bindings,
  toml: wapm_toml::Manifest {
  package: wapm_toml::Package {
  - name: format!("{namespace}/{module_name}"),
  + name: format!("{}/{module_name}", namespace.as_deref().unwrap_or("_")),
  version,
  description,
  license,
  