Change the flag --project-manifest to --manifest-path

[MarcoIeni]

This is because the --manifest-path flag also exists in cargo.

Keep the old flag for cli retro-compatibility, but hide it from the help menu.

• also update the github action input parameter
• also update any occurrence of --project-manifest in the codebase (e.g. in error messages)

[amartyadav]

Hi @MarcoIeni, I would like to implement this, and I have started working on it.

I have a questions to clarify -

• I am assuming you do not want any refactoring of functions to follow the --manifest-path name, right? for eg.
  fn optional_project_manifest(&self) -> Option<&Utf8Path> should remain the same and not be changed to
  fn optional_manifest_path(&self) -> Option<&Utf8Path> . Am I correct to assume this?

[amartyadav]

Addressed this issue in PR #1390. @MarcoIeni , please have a look whenever you can. Cheers! 😇

[MarcoIeni]

Am I correct to assume this?

I don't think it's needed to refactor the functions, yes 👍