`nimble install`: support staged installation

[CyberTailor]

Package managers are usually sandboxed (they don't allow direct installation to system root but install to some intermediate directory instead).

This approach works for most common build systems. They

• Have "configure" to find dependencies in the system root
• Install to $(DESTDIR)

But Nimble checks dependencies and installs files in the same command, so I have no idea how to make it work with Portage.

Some Nimble option can be introduced for manual dependency resolution or for installation directory other than nimbleDir.

[CyberTailor]

Moreover, nimble install builds the package even when nimble build has been already run, so there's no way to separate compilation and installation. If it was possible, I'd be able to sed(1) out all "requires" lines in the *.nimble file before installation at least.