Rustpkg needs to support invoking the default build logic in a package script

[catamorphism]

Sub-bug of #5677

In pkg.rs, there has to be a way to invoke the default build logic. Either provide a library function in librustpkg that does this, or make rustpkg automatically invoke it after executing the contents of pkg.rs (with a way to turn that off). I guess I prefer the first option, but don't care that much.

[metajack]

Thinking about this more, I think the first option is preferable if we keep the fn main() { ... } in pkg.rs.

In order to support both pre-build logic and post-build logic otherwise you'd need to either disallow post-build logic without supressing default behavior, or you could have three functions with default implementations: prebuild, build, and postbuild, where normally people are only overiding the first one.

Since keeping main is the easiest path forward and doesn't close any other paths in the future, option one seems to be the way to go.

[catamorphism]

Agreed.