Automatic installation of plugin helper functions

[weberc2]

Right now, most plugins have starlark helper functions that are defined in starlark files in the repo. These are not available to other repos automatically (they need to be manually copied/pasted into the new repo at the right location).

• Bake the helper functions into the go binary
  • Write these "helper functions" in Go and make them builtins or available to the "load" builtin
  • The Go binary bundles the starlark source code as data
    • Writes these to disk in the repo (perhaps as some init subcommand)
    • Executes these functions on the fly as though they were read from disk first (patch the "load" builtin)
  • Any approach will make for larger binaries, especially bundling the raw source code.
    • We could bake them into the plugins and dynamically load the plugins (see the bullet about a plugin arch below)
• Implement some copy/download mechanism (the binary knows how to download the right version of these files)
  • This is weird since the plugins themselves are statically compiled but the helper functions aren't; potentially hard to avoid version incompatibilities between the compiled plugin and the helper
  • This makes a lot more sense if we dynamically load plugins (i.e., plugins are actually plugins), and a plugin architecture might be a nice feature for extending the platform (related to #28)

[weberc2]

I think the right thing here is to bake these helpers into the plugins themselves. In the future, we can solve for binary size by making the plugins true, dynamically-loaded plugins if necessary. Each plugin can choose whether to provide a Go builtin function or a script that is executed like other starlark code.