package_buildpack() and package_crate_buildpack() contain asserts! even though they return Results

[edmorley]

Currently these functions mix and match assertions and returning Result:

libcnb.rs/libcnb-test/src/build.rs

Lines 15 to 27 in 2698a71

	pub(crate) fn package_crate_buildpack(
	cargo_profile: CargoProfile,
	target_triple: impl AsRef<str>,
	cargo_manifest_dir: &Path,
	target_buildpack_dir: &Path,
	) -> Result<PathBuf, PackageBuildpackError> {
	let buildpack_toml = cargo_manifest_dir.join("buildpack.toml");
	assert!(
	buildpack_toml.exists(),
	"Could not package directory as buildpack! No `buildpack.toml` file exists at {}",
	cargo_manifest_dir.display()
	);

libcnb.rs/libcnb-test/src/build.rs

Lines 74 to 78 in 2698a71

	assert!(
	root_node.is_some(),
	"Could not package directory as buildpack! No buildpack with id `{buildpack_id}` exists in the workspace at {}",
	workspace_root_path.display()
	);

These were introduced as part of #666, and are part of the reason that the regression in #704 was able to sneak in (since it means the tests circumvent the top level error handling that was broken in #666).

They should be converted to additional error types and returned as an error instead.

There's also a lint we can use to prevent this in the future:
https://rust-lang.github.io/rust-clippy/master/index.html#/panic_in_result_fn
(in addition to panics, it detects assertions in result functions)