Prevent multiple PackagingSpecs having same installation path

This fixes a bug that different PackagingSpecs having the same
installation path were silently allowed. Previously, a PackagingSpec
that comes the first for the given installation path won, effectively
eclipsing other PackagingSpecs destined for the same installation path.

Bug: 335506668
Test: go test ./...
Change-Id: Ia36f656e8364f95c4be78fff6e9dc16966307526