build_libcnb_buildpacks_dependency_graph() displays the wrong error for buildpack.toml IO and TOML parsing errors

[edmorley]

The determine_buildpack_kind() function currently returns Option<BuildpackKind>:

libcnb.rs/libcnb-package/src/buildpack_kind.rs

Lines 6 to 8 in 2698a71

	pub fn determine_buildpack_kind(buildpack_dir: &Path) -> Option<BuildpackKind> {
	read_toml_file::<BuildpackDescriptor>(buildpack_dir.join("buildpack.toml"))
	.ok()

Unfortunately, this means it treats "this isn't a buildpack" the same as "an error occurred trying to work out what type of buildpack this is" (which in practice can be IO errors or TOML parsing errors).

This has the knock on impact of the filter here (inside build_libcnb_buildpacks_dependency_graph()) skipping over buildpacks with malformed TOML:
https://github.com/heroku/libcnb.rs/blob/2698a713edf19034e382aa702e12acc1516da8fa/libcnb-package/src/buildpack_dependency_graph.rs#L36C1-L41

Which means that the packaging process continues on further than it should, and then a confusing error message is shown instead of a "unable to parse TOML" error:

Error while creating dependency graph: Node references unknown dependency libcnb-test/a