The current check only validates that an element that specifies
"element-provides" doesn't conflict with a "real" element. We also
want to check this against the provides of other elements.
A real example is with a "block-device" element. There is no actual
"block-device" element; we can have multiple elements provide it
(block-device-[gpt,mbr,efi], say) but we only want one of them at a
time.
Update the unit test for this.
Change-Id: I59d4aa5f6f09e2892b213e154befa10d85e95ca3
a32a8f1ce1