Support not running installers if a package is already present

[ibarakaiev]

Describe the bug
Right now, if a package is already installed, Igniter outputs that the dependency is already in mix.exs but proceeds to run the installer:

Dependency ash_postgres is already in mix.exs with the desired version. Skipping.

The following installer was found and executed: `ash_postgres.install`:

    No proposed content changes!

The following tasks will be run

* ash.codegen initialize

To Reproduce
Run mix igniter.install ash_postgres twice.

Expected behavior
To avoid inconsistency (i.e. saying "skipping" then proceeding to install), if a package is already installed, I'd suggest either:

• Skipping the installation as well by default, but supporting a flag like to run the installer anyway even if present.
• Avoid saying "Skipping" by default, always running the installer, but support --skip-if-present which would skip the installer if passed.

[zachdaniel]

🤔 we could make it interactive like "Dependency ash_postgres is already in mix.exs, would you like to rerun the installer"?

[ibarakaiev]

That's a great idea from the DX perspective—doesn't require remembering what flag to use. I wonder what should the behavior be when --yes is passed in this case, though 🤔 Would probably still require a flag to overwrite it in case the end user wants to auto-accept everything but not run installers unless a new package is being added (that's the use case in Fireside, for example).

[zachdaniel]

--yes should automatically reply yes to any questions. The implication behind it is that it makes igniter scriptable. I.e "yes I know what this command will do and I want you to do it". So we could add a flag like you mentioned that makes it not ask, assuming false. Like --skip-installed