Improve the `unusedDependencies` lint rule to check for products targets

[marinofelipe]

Currently, the unusedDependencies lint rule, which comes out of the box with spmgraph, simply checks for usages that match a linked product dependency without checking the available targets that come with such product.

For most cases, it works just fine because it's common practice for products to have a single target that matches the product name, i.e.

.library(
   name: "FirebaseAuth",
   targets: ["FirebaseAuth"]
),

However, for products where either

• The product name doesn't match the target; or
• There are multiple targets available

The linter looks for imports of "$productName", so it wrongly considers it a failure.