Explain why a module is in a project's dependency graph.

Usage:
  v why [PACKAGE]

With no PACKAGE, prints the whole graph, rooted at the current project.
With a PACKAGE, prints only the routes that reach it.

Examples:
  v why                 # the whole graph
  v why vsl             # why vsl is here

The graph is built by reading each installed module's own v.mod, so PACKAGE
may be written either as a registered name or as a git URL. Modules are looked
up in the project's own folder (where `v install --local` puts them) and then
in VMODULES. A dependency that is required but not installed is reported as
such rather than left out.

`v why` follows the `dependencies` declared in v.mod files; to see which imports
bring a module into the build, use `v mod why`.

This command only reads. It never contacts the registry or the network, so it
works in a build that has no network at all.

Options:
  -v  Print more details about the performed operation.

Versioned requirements are annotated with each parent's constraint and the
installed version. `v mod graph` prints these manifest dependencies as flat edges.
