These changes extend the public API of `pkg/codegen/schema` to support
on-demand binding of package members. On-demand binding is appropriate for
scenarios that do not require the entire package, especially those such as
program code generation or the YAML LSP server that require only specific
types/functions/etc.
The extensions to the public API consist of two new types and several new
methods. The most notable of these are `PackageReference` and
`Loader.LoadPackageReference`. The former provides the on-demand binding
interface, while the latter creates instances of the former (n.b. it was
my intent to make a breaking change to the signature of `Loader.LoadPackage`
s.t. it returns a `PackageReference`, but the circular dependency between
this Go module and those for YAML and Java prevented that change).
These changes _dramatically_ reduce the memory required to interace with
Pulumi Packages, and only require memory proportional to the number of
accessed package members. We may be able to improve on this in the future
by removing type/resource/function interning, which would allow those
values to be garbage collected at a granaular level rather than at a
package level. That is a more radical change, though, as it requires new
equality semantics for each of the affected types (some of which are
currently used as map keys).