Spirv builder cleanup

[Firestar99]

Requires #484#488#496

• review commit by commit after the merge commits
• just some general cleanup
• were originally created for spirv-builder: when no artifact is produced, error instead of panic #496, but replaced it with a far simpler implementation, but these cleanups are still good

[LegNeato]

It would be better to have two variants here for the single and multi-module case to make the message more explicit and the message better?

[Firestar99]

The error message on main is completely wrong: There are no "multi-module" metadata files. The metadata file is needed regardless of the module being a multi-module or single module, as it contains all entry point names and *.spv paths which are returned to you after building.

In fact there are four cases MetadataFileMissing will be emitted on main:

• parse_metadata_file: The metadata file is missing, stays MetadataFileMissing. Can also fail with MetadataFileMalformed.
• leaf_deps fails to read rustc dependency information files, now DepFileParseError.
• query_rustc_version fails to query the rustc version, now generic IoError.
• TargetSpecVersion::target_arg fails to write the target spec file, now generic IoError.

The new error messages:

rust-gpu/crates/spirv-builder/src/lib.rs

Lines 120 to 125 in a292dc3

	#[error("Metadata file emitted by codegen backend is missing: {0}")]
	MetadataFileMissing(std::io::Error),
	#[error("Metadata file emitted by codegen backend contains invalid json: {0}")]
	MetadataFileMalformed(serde_json::Error),
	#[error("Couldn't parse rustc dependency files: {0}")]
	DepFileParseError(std::io::Error),

rust-gpu/crates/spirv-builder/src/lib.rs

Lines 135 to 136 in a292dc3

	#[error("IO Error: {0}")]
	IoError(#[from] std::io::Error),

[Firestar99]

Oops, find replace was a little too aggressive there... squashed it in

[Firestar99]

Force-merged since review from LegNeato is blocking it