Automatically detect 'depends' by inspecting the included library

[virtuald]

• On Linux, pyelftools can be used to find library dependencies
• On Windows, pefile can be used to find DLL dependencies
• On OSX, delocate can be used to find dylib dependencies

An implementation of this would figure out the dependency at build time by using pkgcfg to figure out what robotpy-build libraries are already available. The relink_libs functionality already has the code to do much of the pkgcfg stuff since it has to do exactly this.