Add dependencies of vendored deps to ligo.opam

This change modifies ligo.opam to contain all the dependencies needed to compile ligo.

Motivation

The dependencies of the vendored dependencies, I think, belong in the top level .opam. It's kind of suboptimal, because it'd be nice if they could be kept next to the vendored dependencies themselves, but it's important that there be a single .opam that contains all the dependencies needed by the project, or you run into issues when trying to depend on ligo from another package (the package manager can read the .opam you point it to and download all the dependencies, but it won't know to crawl through the vendored-deps and download their dependencies too.) Previously, the dependency-set was kind of informally specified as "ligo.opam, plus the dependencies of the output of find vendors -name \*.opam", which is complex enough to really complicate the CI as well as the lives of anyone trying to use ligo as a library from another project.

(To keep this PR simple I haven't modified the CI or build scripts at all, but I'll do that next as @lesenechal.remi asked. But to summarize what will happen: we no longer have to treat the vendors folder in any special way, and never need to write $(find vendors -name \*.opam) again.)