Support for dependency path relative to the fpm.toml it’s written in

[zoziha]

To close #605 .

Description

The previous dependency path was always quite the same as the top-level project, which made it difficult to make such an fpm package called by a higher-level project.
Rust-cargo seems to take a more reasonable approach: dependency paths are relative to the Cargo.toml it’s written in.

This PR attempts to support such improvements, which will allow us to publish fpm packages with component libraries.

---- pkg-1
      |---- fpm.toml
      |---- crate
              |---- pkg-2
              |---- pkg-3

# pkg-1 fpm.toml
[dependencies]
pkg-2 = { path = 'crate/pkg-2' }

# pkg-2 fpm.toml
[dependencies]
pkg-3 = { path = '../pkg-3' }

What this PR does

• Update the interfaces of dependency.f90::new_dependecy and dependency.f90::new_dependecies, adding the root optional argument.
• Add an example;
• Update test and help manual;
• Update README for MSYS2-fpm. (Not related to this PR, but a minor revision)
• Update the handling of join_path("", "fpm.toml"):

print *, join_path("", "fpm.toml")  !! Original: `/fpm.toml`
print *, join_path("", "fpm.toml")  !! New:      `fpm.toml`

• On Windows, adapt the path style of the Windows system to the dependency path.

[LKedward]

Hi @zoziha, apologies for the late review of this! This all looks good to me, thanks a lot! 👍

[zoziha]

Thanks @awvwgk and @LKedward for reviewing.
With 2 approvals, maybe this can be merged next.

[awvwgk]

@zoziha Sure, feel free to go ahead and merge.