Prevent clashes between template fields and stream/input fields

Consider the following:

`manifest.yml`
```yml
policy_templates:
- name: apm-server
inputs:
- type: apm
vars:
- name: name
default: my-default-name
template_path: ./agent/input/template.yml.hbs
```

`agent/input/template.yml.hbs`
```yml
name: {{name}}
```

There will be a clash between the `name` field in the template (package-defined) and the `name` field in the input (spec-defined)

Same thing occurs for data streams templates. This scenario should be detected and avoided.