CLi: made cli-lib modules publicly available for other crates

[silvestrpredko]

Made modules database, metadata, ... publicly available for users that want to include sqlx-cli into their own helper binary or other library. (It's easier to use migrate::run directly without building Opt).

PR doesn't contain any significant changes.

[abonander]

sqlx-cli is not meant to be used as a library. I don't want to have to think of downstream API users when making changes to it.

[silvestrpredko]

sqlx-cli is not meant to be used as a library. I don't want to have to think of downstream API users when making changes to it.

There is nothing wrong with opening these modules; even if there will be breaking changes, it's not a problem for the project, I think.

For now, I want to have a custom behaviour for certain operations with migration revert, and I need to write all the logic for the rest of the CLI by myself, or build a whole Opt.

[abonander]

This needs the same semver-exempt notice as sqlx-core:

sqlx/sqlx-core/src/lib.rs

Lines 3 to 14 in 764ae2f

	//! ### Note: Semver Exempt API
	//! The API of this crate is not meant for general use and does *not* follow Semantic Versioning.
	//! The only crate that follows Semantic Versioning in the project is the `sqlx` crate itself.
	//! If you are building a custom SQLx driver, you should pin an exact version for `sqlx-core` to
	//! avoid breakages:
	//!
	//! ```toml
	//! sqlx-core = { version = "=0.6.2" }
	//! ```
	//!
	//! And then make releases in lockstep with `sqlx-core`. We recommend all driver crates, in-tree
	//! or otherwise, use the same version numbers as `sqlx-core` to avoid confusion.