[Sugar-devel] Check for required activity deps before installing

[Aleksey Lim]

It is a mimic of sweets' requires tag that will be used only for sugar
dependency. Full functional dependency tracking will come with sweets support
implementation in the shell.

Dependency check will happen only for installation. Since in most cases
it should be enough and launching code is scattered among the shell code
(porting the 0.90 patch that fixes it, might be invasive for dx-2).