Add a command (maybe called spack mark) that lets you mark a package as implicit. The command could also be extended to do the inverse (which currently can be accomplished by installing the exact package spec again).

Rationale

I was "manually" installing a bunch of packages that were failing to build whilst creating the package for kitty. So now I have a bunch of packages that are really implicit dependencies on kitty that are marked explicit and show up when I run spack find -e.

Description

A command that would mark in Spack internal db that a package is in fact implicit, rather than explicit (or vice-versa). Strawman example uses: spack mark implicit [email protected], spack mark explicit pkg-config.