Layer should not contain absolute path

[qfox]

Atm BemCell.layer contains path to blocks directory. But layer is an Id of layer.

E.g. for classic scheme (${level}/${layer}.blocks/${entity.id}.${tech}) we have paths:

/levels/bem-core/common.blocks/page/page.css
/levels/bem-core/desktop.blocks/page/page.css

For modern scheme (${level}/${entity.id}@${layer}.${tech}) we have paths:

/levels/bem-core/blocks/page/page.css
/levels/bem-core/blocks/page/[email protected]

It means we should not include any absolute paths into layer and should provide level as a property of BemFile while BemCell should contains pure layer id.