This information is useful when you want to build several SyntaxSets, but
without having to duplicate SyntaxDefinitions. For example:
"Rust" has no dependencies. But "Markdown" depends on "Rust". With the data
structures this code adds, we know that "Rust" is a dependent syntax for
"Markdown", and can construct a SyntaxSet that takes that into account.
Note that code has a temporary environment flag to ignore any information about
dependents when constructing SyntaxSets. Code that makes use of the new data
structure will be added later.
