Dependency specification without version constraint

[taichi-ishitani]

In case of specifying dependencies on local storage, such dependencies may be under development and version control is not suitable for them.
I think it would be nice to allow to specify such dependencies without version constraint and import existing source files in the dependencies as is.

[dalance]

I thought an actual workflow like below:

• develop a project using local path dependency
• publish the dependency project by new version
• publish the project by new version

In this case, both local path and version is required.
So the following specification is probably reasonable.

[dependencies]
"https://github.com/veryl-lang/sample" = { version = "0.1.0", path = "../../sample" }

This means "If path exists, it is used as is, and if not, version is used from the source repository".

[dalance]

I implemented this feature at #1206.
Now the following path specification can be used.

[dependencies]
"https://github.com/veryl-lang/sample" = { version = "0.1.0", path = "../../sample" }
"../../sample"                         = { version = "0.1.0", path = "../../sample" }

In this PR, there are the following restrictions:

• left-hand side URL/path is mandatory. It should be valid git repository.
• version is mandatory. It should be published.

These restrictions are required to keep consistency with the existing lockfile system.
How about it?