'@publish import' and explicit control of the host visible shader interface

[mighdoll]

From discord discussion:

WGSL statements that change the host visible interface of the shader, like override or @compute, should only be linked if they're referenced from the root module (e.g. ./main.wesl).

Host visible names are: override, entry points (@compute, @vertex, @frag), and resource binding numbers (@group(0) @binding(1))

• host visible names are only linked as host visible names if they are:
  • declared in the root module, or
  • if they are explicitly mentioned in the root module with in an @publish import statement like:
    • @publish import super::util::over;

Error conditions:

• it's a link error to use @publish import in any module other than the root module.
• override elements declared in non-root modules and not imported via with @publish import are converted to const elements. (or simply dropped if they are not referenced even indirectly from a root module element)
• it's a link error if a non-root module resource binding is referenced indirectly from a root element but not made visible with @publish import in the root module. (but it's ok to have non-root module resource binding that's unreferenced.)