SourceUnitTypes is a list of source unit types (e.g., "GoPackage") that this tool can operate on.

If this tool doesn't operate on source units (for example, it operates on directories or repositories, such as the "blame" tools), then this will be empty.

TODO(sqs): determine how repository- or directory-level tools will be defined.

SourceUnitTypes is referenced in 3 repositories sourcegraph/srclib