Introduce a package resolver

[Michael-F-Bryan]

Description

This adds a PackageResolver getter to WasiRuntime and wires it up to the ModuleCache.

TL;DR:

• The WasiRuntime trait now has a package_resolver() method which gives you an Arc<dyn PackageResolver>
• A PackageResolver will resolve package identifiers to a BinaryPackage (expect the return type to change - eventually I want to return a wasmer_wasix::runtime::resolver::ResolvedPackage)
• The PluggableRuntime will use the "builtin" resolver from lib/wasi/src/runtime/resolver/builtin.rs by default, this just calls into the existing wasmer_wasix::wapm::fetch_webc() code which uses the WAPM backend and will cache *.webc files to disk
  • Both the cache directory and URL for the WAPM backend are configurable
  • there is a BuiltinResolver::from_env() constructor that will eventually set things up using $WASMER_HOME/wasmer.toml (I've just added a quick'n'dirty implementation for now)
• The ModuleCache::get_webc() module now just defers to runtime.package_resolver().resolve_package(...) because the PackageResolver implementation should be in charge of caching
• Now that ModuleCache doesn't keep the in-memory cache of resolved BinaryPackages I've added a with_cache() method that returns InMemoryCache<Self> so you can wrap an existing PackageResolver with an in-memory caching layer
• Removed the --include-webc flag from wasmer run because it used to explicitly add items to the ModuleCache's WEBC cache and all that code has been deleted (not sure if this should have been a hard delete or if we should warn people when they use it that --include-webc is now a no-op)
• Removed the engine() getter from the WasiRuntime trait so we can introduce a separate mechanism for module compilation and caching (nothing used this anyway, they just called WasiRuntime::new_store())

CC: #3603.

[syrusakbary]

(let’s not merge until we release Wasmer 3.3)