Allow specifying a local project as a build dependency

[NicklasXYZ]

Build tool feature request: Allow specifying a local project as a build dependency.

For example, I've previously used the following with rebar3 to specify a local project local_project as a dependency:

{deps, [
    {local_project, {path, "local_project"}},
]}. 

It would be cool if the new Gleam build tool supported this kind of build pattern as well.

I hope I am not creating a duplicate issue :-)

[lpil]

A super useful feature, let's do this for sure.

Related to #1338 as it'll share a lot of the same code.

[lpil]

Some things we'll need to do:

• Work out how it fits into the build directory
• Integrate it into dependency version resolution
• Ban publishing of packages that have non-hex deps

[paulo-ferraz-oliveira]

@lpil, you probably know this already, but in rebar3 you can achieve the same effect by using folder _checkouts (think symbolic links) in which case rebar3 will signal the dependency as local (checkout) and will not update rebar.lock with its reference.

(was brought here from the Erlang Forums)