Make sea-orm-cli & sea-orm-migration dependencies optional

[xamgore]

Addresses #2320.

My team is working on a workspace project. To make sure generated models are always the same, we decided to make sea-orm-cli a member crate. You may find details at the discussion 1889.

The problem is, sea-orm-cli/codegen feature imports all the database drivers, even though sqlite and mysql represent zero interest for me.

codegen = ["sea-schema/sqlx-all", "sea-orm-codegen"]

libsqlite-sys + sqlx-mysql + sqlx-sqlite take quite a few seconds in total compilation time, and I believe this could be improved.

I've gone through dependency-hell feature-shenanigans, but could easily make a mistake. So testing and merging this PR won't be easy. I tested the code on my local environment: tweaked all the possible combinations of features to see, whether the compiler would grumble. Also, I have patched my team project with local sea-orm codebase and tested it against it.

[xamgore]

@billy1624 could you allow CI checks, please?

[xamgore]

I've rebased the branch, probably some issues should be gone now.

[tyt2y3]

would something like this work?

        #[cfg(feature = "sqlx-mysql")]
        DbBackend::MySql => MySql.query_tables(),
        #[cfg(feature = "sqlx-postgres")]
        DbBackend::Postgres => Postgres.query_tables(),
        #[cfg(feature = "sqlx-sqlite")]
        DbBackend::Sqlite => Sqlite.query_tables(),

if the feature is not enabled, the variant shouldn't exist

[xamgore]

Unfortunately not, as Rust needs the arms to be present. But the direction of thought is right, thank you

[xamgore]

Ok, I've started testing against these two checks from the action scripts:

cargo clippy --manifest-path sea-orm-migration/Cargo.toml -- -D warnings
cargo clippy --manifest-path sea-orm-migration/Cargo.toml --features sqlx-postgres -- -D warnings

+on my personal project which uses the patched version

[patch.crates-io]
sea-orm = { git = "https://github.com/xamgore/sea-orm.git", branch = "master" }
sea-orm-cli = { git = "https://github.com/xamgore/sea-orm.git", branch = "master" }
sea-orm-migration = { git = "https://github.com/xamgore/sea-orm.git", branch = "master" }

It works.