auto add workspaces to root package.json

[GiladShoham]

If yarn workspaces is enabled we should validate that the components dirs are configured in the workspaces in the root's package.json
During import we should make sure that the paths:
componentsDefaultDirectory
dependenciesDirectory
are defined.
If we do import with -p we should add this path to the workspaces

We will add a new key to bit.json called manageWorkspaces which is boolean define wether to auto add workspaces to package.json

• add field to bit.json manageWorkspaces
• add imported components to package.json (workspaces field)