BaseTools/Workspace: Fix ValueChain set

BaseTools/Workspace: Fix ValueChain set

Commit 88252a90d1ca7846731cd2e4e8e860454f7d97a3 changed ValueChain
from a dict to a set, but also changed the (former) key type from a
touple to two separate values, which was probably unintended and also
breaks build for packages involving Structured PCDs, because add()
only takes one argument.
This commit changes the values back to touples.

V2:
  - Removed a whitespace change.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Marvin Haeuser <Marvin.Haeuser@outlook.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>