select_file() build rule implementation.
Selects a single file from the outputs of a target by given relative path.
## select_file
select_file(name, srcs, subpath)
Selects a single file from the outputs of a target by given relative path
**ATTRIBUTES**
| Name | Description | Type | Mandatory | Default |
| :------------- | :------------- | :------------- | :------------- | :------------- |
| name | A unique name for this target. | Name | required | |
| srcs | The target producing the file among other outputs | Label | required | |
| subpath | Relative path to the file | String | required | |