Support Git-based versioning in epm

[giancosta86]

Elvish is a superlative shell, always surprising me with its excellent quality and minimalist flexibility - which is why I feel now the need for extending it with semver-based package versioning, at least for Git repositories, so as to be able to handle the evolution of software architectures over time.

The mechanics of the present pull request are thoroughly described in the README of my epm-plus project, which contains the very same code as the PR, plus a few lines to patch epm at runtime.

Please, find attached a suite of proof-of-concept tests: they are based on my velvet test framework and runner for Elvish, and can be run via the verify.elv script:

elvish pkg/mods/epm/verify.elv

from the Elvish project directory.

[giancosta86]

I've just added - via a dedicated commit - the concept of metadata-driven install, as well as the related devDependencies field for metadata.json; the details and the rationale for both are described, as usual, in the documentation for my epm-plus project.