Consider accepting nuget metadata from non-cargo sources

[KodrAus]

It turns out this library is a fairly handy little tool for collecting platform-specific libraries into a single Nuget package. The cross command doesn't make any assumptions about how those platform-specific libraries are produced. If we accepted metadata from other sources besides a Cargo.toml (possibly even still using TOML as a format) then we'd be able to neatly package up libraries with no Cargo integration at all.