Improved Concurrency when building workspace and packages

When using dune to build packages, the build plan must proceed as follows:

1. Build all packages defined in dune.lock
2. Build everything defined in the local workspace

These two steps must be done in sequence because to build something in the local workspace, dune must resolve the libraries being used. To resolve any particular library, dune needs to build all packages. This is because dune has no idea in which package any particular library must exist.

Ideally, we'd want 1. and 2. to run concurrently. One way to do that is to include additional metadata in opam files about the libraries that a package might provide. The following two constructs might be particularly useful:

x-findlib-library-mask: [
  "bar" # the library will always be provided
 "bar.*" # any number of libraries under the namespace bar. might be provided
 "foo?" # the library foo might be optionally provided
]

This information will be sufficient for dune to only build the subset of packages that might contain a particular library.

Start by tracing how dune resolves libraries and sequences builds for packages in dune.lock and the local workspace. Review the proposed opam metadata, including library masks and optional libraries, and determine how it could safely support concurrent planning. Done means the two build phases can overlap without breaking library resolution, with coverage for the resulting build plan.