feat: disallow nested packages

[kayagokalp]

Description

closes #4072

Disallows package structure such as the following:

proj1
├── proj2
│   ├── Forc.lock
│   ├── Forc.toml
│   ├── out
│   └── src
├── Forc.lock
├── Forc.toml
├── out
│   └── debug
└── src
    └── main.sw

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).
• 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.

[kayagokalp]

blocked by #4412

[mitchmindtree]

Nice, this will be handy! Just a few small suggestions, otherwise lgtm

[mitchmindtree]

Nice! One last nit

[mitchmindtree]

👌