Fix impl self node order dependency pass key collision issue.

[tritao]

Description

This fixes a key collision issue that happened in the node dependency pass while handling impl self nodes with multiple blocks.

See the following example for a simple example of the problem:

struct A {}

impl A {   //dependency of this impl is overwritten by the next impl
    fn a() {}
    fn b() {}
}

impl A {   //this will overwrite the dependency for the previous impl
    fn ab() {}
}

Fixes the problem by introducing a , while joining the names for the key.

Closes #6319.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.