Build Option Expansion (aka "templating")

[rydrman]

Discussed in https://github.com/imageworks/spk/discussions/77

• syntax will be handlebars-style with double curly braces: eg {{ variable }}
• initial scope is to only be able to expand variables that are the current package's build variables
• look at ensuring variable naming is restricted so that we have the option to possibly expand where variables can come from in the future - namely expanding values of variables from build dependencies, eg {{python.abi}}