This adds a package manager for Inko, creatively called "ipm" (Inko
Package Manager).

The package manager doesn't use a centralised package registry, instead
it relies entirely on Git repositories. These repositories can be hosted
anywhere (GitHub, GitLab, locally, etc), as long as the URL is supported
by the Git CLI. For GitHub and GitLab we also support URLs that leave
out the scheme, in which case we translate them to HTTPS URLs.

Packages are specified in the file `inko.pkg`, which must reside at the
root directory of a project. This file is a line based file and lists
the dependencies that are required. Such a file might look as follows:

    require github.com/alice/json 1.2.3 abcdef123
    require gitlab.com/inko-lang/foo 1.3.0 ffee465

Adding and removing entries is done using the commands `ipm add` and
`ipm remove respectively`. To install the dependencies into your
project, you run `ipm sync`. The result is a ./dep directory containing
all your dependencies. The compiler automatically adds this directory to
the load path, if it exists. Updating dependencies is done using `ipm
update`.

The package manager uses minimal version selection, as this is trivial
to implement, leads to predictable builds, doesn't require a SAT solver,
and doesn't require lock files. We don't allow different major versions
of the same package, as this can lead to subtle bugs when data is passed
between these versions.

I opted to make package management a separate process/tool as this makes
it easier to build packages without an internet connection, and because
it's easier to maintain. Packages are installed in a local ./dep
directory because this makes it easier for the compiler to load the
dependencies, and makes packaging a project including all its
dependencies trivial (= just archive the entire project and off you go).

In terms of features the package manager provides, it's quite basic.
Most notably, we don't support overriding packages with local copies at
this time, and error/progress reporting could be improved in various
places. Testing is partially done by hand, as automatic testing of the
various CLI components is a bit tricky due to heavy network usage. We
may improve this in the future.

Changelog: added