Allow importing BUILD files

[weberc2]

Rather than using reftarget() to address other targets from BUILD files, we should allow importing/loading them and referencing them using their symbols in the starlark language.

Considerations

• What if we remove the name = ... field and only use they name of the symbol in the defining package?
• What if mktarget() doesn't actually mutate the list of targets, but maybe we collect targets by scanning the globals output from each package?
  • How do we know what was the original defining package?
  • How do we handle cases where there are two global symbols that reference the same target? E.g., a = mktarget(...); b = a? Are a and b two distinct targets (identical in everything but name)?

[weberc2]

This is largely addressed by cb8a0eb0e999f09e0fa1bca863c7d274c958438a. There is now a distinction between loading packages (BUILD files) and loading libraries (.star files). Libraries must be loaded via load("lib://path/to/lib", ...) where the path must point to a .star file. Libraries may also load other libraries but not targets, and libraries may also not invoke mktarget directly. This may change in the future, especially concerning libraries that would like to add a dependency (e.g., the py_test function creates Python test targets which should each depend on the pytest target).