You have two main kind of dependencies:
- source dependencies (you need to include, within the sources of your project, source from another project),
- binary dependencies (you need to include a packaged set of files, like the ones found in a shared library).
If, when you say "I use these libraries in a project", you mean you need the binaries in order for your project to compile, then you could store said binaries in an external repository (i.e. not a (D)VCS like Mercurial, but an artifact repository like Nexus)
But if you mean you need to include sources, because you are also making some evolutions to those libraries while using them to develop your project, then Mercurial subrepos are a better fit.
