Force unique package

[janmasrovira]

Context

Currently it is allowed that the dependency tree of a project depends trasitively to the same package with different versions. Most often than not, this causes confusing errors or even crashes.
The use of the defaultStdlib option in Package.juvix often results in having different versions on the standard library in the same dependency tree, which causes the problems mentioned above.

Proposal

This proposal was sketched in a conversation with @paulcadman and @lukaszcz

Remove defaultStdlib

This option will be removed. The standard library will no longer be bundled with the compiler.
In the global package we will put a dependency of the most recent version of the standard library. Likewise with defaultPackage.

Do not allow different versions of the same package

The transitive dependency tree must have at most one version of each package.

Allow dependency overwriting

We would add a field to the Package.juvix that would associate packageName : String to packageVersion : SemVer.

The overwrite preferences of the parent package take precedence.

Add minimal compiler version field

We would add a field that indicates what's the minimal juvix version supported for that Package.

[janmasrovira]

Approved