Stack elements permit `build-depends`

Summary

Stack elements permit build-depends field.

Steps to reproduce

Create elements with the following configurations

./stack.bst

kind: stack
build-depends: dependency.bst

./dependency.bst

kind: import
source: 
- kind: local
  path: project.conf

and run bst build stack.bst

What is the current bug behavior?

Currently there is no error being thrown at stack.bst for having a build-depends field. Having build-depends as a field is incorrect as running bst artifact checkout stack.bst will show that the artifact produced is empty rather than containing project.conf.

What is the expected correct behavior?

Attempting to bst build a stack element that contains build-depends should fails as it is semantically nonsensical. Instead an error message should be printed suggesting perhaps to change build-depends to depends.

Possible fixes

Set BST_FORBID_BDEPENDS to True for the stack element.