support packages sourced from local filesystem, not just remote urls

[Arnaz87]

When developing and testing packages, they are not saved in github but instead are in the local machine of the developer, machu picchu needs a way to link to those versions of the packages.

• Peru has peru sync [package] [directory]
• In Cargo a path option is added to the dependency. It currently has no way of not modifying the cargo file so it has to be reverted manually before pushing
• npm has a link command that when run without arguments, creates a package link in the current machine, and when run in another project, npm link [package] creates a symlink to the previously linked package as the dependency

I like the idea of a manifest.patch.scl file that is untracked by git that overrides the manifest file, along with an option for an mp package to point to a directory rather than a git link.