Use Label (string) instead of Label (struct) to dedupe

Using Label struct as the map key causes issues because it contains
OriginalModuleName. The same module will have a different value for this
property when called from inside a soong namespace vs from outside.

If there are dups, we can just choose the first one. OriginalModuleName
is often used with ModuleFromName, and that function panics if there are
modules with the same name in two separate soong namespaces

Test: go test ./bp2build ./bazel

Change-Id: I2ee33efab03016a72f1eea99cb958b6198baeca2