Public data in plugin yaml is ignored

When writing a new plugin e.g.:

my-plugin.py
```
from buildstream import BuildElement

class MyPluginElement(BuildElement):
BST_MIN_VERSION = "2.2"

def setup():
return MyPluginElement
```

my-plugin.yaml
```
variables:
my_var: "some value"

config:
install-commands:
- echo "Installing a file to %{install-root}"
public:
bst:
split-rules:
integration-commands:
- echo "integrating artifacts"
devel:
- /my/custom/development/paths
```

The public section of the YAML is ignored by Buildstream.

Instead it's required to go via the python API:

```python
from buildstream import BuildElement

class MyPluginElement(BuildElement):
BST_MIN_VERSION = "2.2"

def assemble(self, sandbox):
collectdir = super().assemble(sandbox)

bstdata = self.get_public_data("bst")
bstdata["integration-commands"] = [ 'echo "integrating artifacts" ' ]
bstdata["split-rules"] = {
"devel" : [ " /my/custom/development/paths" ]
}
self.set_public_data("bst", bstdata)

return collectdir

def setup():
return MyPluginElement
````

Proposed solutions from @gtristan

It should either (A) Be supported or (B) Trigger a validation error at load time

Compare the public section in my-plugin.yaml with the get_public_data("bst") and set_public_data("bst", ...) path shown in my-plugin.py. Start by loading the example plugin and trace how its YAML is handled. Done means the public YAML is honored, or loading it produces a clear validation error.

Written by the indexing model from the issue text.