Allow WIT files to import other WIT files

[Michael-F-Bryan]

At the moment, the wapm CLI expects each set of bindings to be completely self-contained, however it's not uncommon for one WIT file to import types or functionality from another. When packaging a WAPM package, the wapm CLI should recursively include any bindings that are used.

For example, it’s pretty common for the functionality exposed by the host and the guest to share the same types (e.g. errno in WASI).That means you’ll have three WIT files:

• guest.wit
• host.wit
• common.wit

... where both guest.wit and host.wit import records/resources/whatever from common.wit.

At the moment, the only WIT file that gets added to the bundled *.tar.gz is the one specified in your wapm.toml. Therefore, if we tried to generate bindings for a package like the one above, wit-pack would error out with “unable to import common.wit: file not found”.