Option -h doesn't show description for imported targets

[fizyk]

I've got a magefile targets placed in thematical modules, and one common target in main magefile.

The main target's description with -h file shows just fine (sans #358 )
howeverm for any imported target I get the Unknown target: "target:name" message. Even though the target with first line description is listed just fine on a list.

[fizyk]

works with the most recent release