Add a Target to the build script deps key

Add a Target to the build script deps key

Previously a Target was not considered to be in the key of the map to build
script dependencies, but this meant that information like whether it was a
dev-dependency was lost. As a result libraries would depend on the build scripts
of their dev-dependencies, causing an internal error when the build script
hadn't finished yet when the library was being built.

Closes #1751