Repository: go
Issue: encoding/xml: does not check namespace constraints that do not require keeping extra state
Labels: NeedsInvestigation

### Go version

`go version go1.21.11 linux/amd64`

### Output of `go env` in your module/workspace:

```shell
GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/user/.cache/go-build'
GOENV='/home/user/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/user/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/user/go'
GOPRIVATE=''
GOPROXY='direct'
GOROOT='/usr/lib/golang'
GOSUMDB='off'
GOTMPDIR=''
GOTOOLCHAIN='local'
GOTOOLDIR='/usr/lib/golang/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.21.11'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/dev/null'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build3671854511=/tmp/go-build -gno-record-gcc-switches'
```

### What did you do?

https://go.dev/play/p/FRIvYTU13fC

### What did you see happen?

`encoding/xml` does not reject XML with the following violations of namespace constraints:

1. Binding a URL other than `http://www.w3.org/XML/1998/namespace` to the prefix `xml`:

    ```xml
    <!-- Ill-formed: cannot bind prefix xml: to a URL other than
         http://www.w3.org/XML/1998/namespace -->
    <a xmlns:xml="https://example.com"/>
    ```

2. Binding `http://www.w3.org/XML/1998/namespace` to a prefix other than `xml`:

    ```xml
    <!-- Ill-formed: cannot bind a prefix other than xml: to
         http://www.w3.org/XML/1998/namespace -->
    <a xmlns:a="http://www.w3.org/XML/1998/namespace"/>
    ```

3. Declaring `http://www.w3.org/XML/1998/namespace` as the default namespace:

    ```xml
    <!-- Ill-formed: cannot declare http://www.w3.org/XML/1998/namespace
         as default namespace -->
    <a xmlns="http://www.w3.org/XML/1998/namespace"/>
    ```

4. Declaring the prefix `xmlns`, whether it is with the correct URL `http://www.w3.org/2000/xmlns/`

    ```xml
    <!-- Ill-formed: cannot declare xmlns: prefix -->
    <a xmlns:xmlns="http://www.w3.org/2000/xmlns/"/>
    ```

    or with any other URL:

    ```xml
    <!-- Ill-formed: cannot declare xmlns: prefix -->
    <a xmlns:xmlns="https://example.com"/>
    ```

5. Binding the URL `http://www.w3.org/2000/xmlns/` to any prefix:

    
    ```xml
    <!-- Ill-formed: cannot bind a prefix to
         http://www.w3.org/2000/xmlns/ -->
    <a xmlns:a="http://www.w3.org/2000/xmlns/"/>
    ```

6. Declaring `http://www.w3.org/2000/xmlns/` as the default namespace:

    ```xml
    <!-- Ill-formed: cannot declare http://www.w3.org/2000/xmlns/
         as default namespace -->
    <a xmlns="http://www.w3.org/2000/xmlns/"/>
    ```

7. Using `xmlns` as the prefix for an element:

   ```xml
   <!-- Ill-formed: xmlns: cannot be the prefix of an element name -->
   <xmlns:a/>
   ```

8. Undeclaring a namespace prefix by binding the empty string

   ```xml
   <!-- Ill-formed: cannot bind a namespace to the empty string -->
   <a xmlns:a=""/>
   ```

### What did you expect to see?

`encoding/xml` should reject these violations, whether `Token` or `RawToken` are used.  Detecting these violations can be done without maintaining any additional state and without resolving prefixes to namespace URLs.