This is a huge patch that rewrites how the parser receives content and
how the import hook works. Externally, there is no difference. In
terms of API, there are several breaks.

Import hook callers now receive only the state and target. The old
`lily_load_*` functions are now `lily_import_*` functions. The
renamed functions automatically tack on the appropriate suffix. This
allows useful hooks without even looking at msgbuf!

New functions `lily_import_use_local/package_dir`. These functions
control if the import functions are going to use a local dir or the
package dir. The caller can specify a directory that fits between the
package root and the target. This allows building a system that has
tests in a `test` directory that can cross-import `src` files.

New function `lily_import_current_root_dir` added. It omits the
leading directory to the first module. That seems right. It's for the
testing system that's yet to be built (but which is now POSSIBLE!).

Content loading has also changed. It's done through `lily_load_*`
functions (newly-made, unrelated to old). The embedder can then call
a parse/render function. The advantage of this is that it decouples
parse/render from their content (render_file was hard to test for
example).

All embedders are broken by this patch. Sorry.