Extracted from #14265.

Terminology clarification: #14307

Add subcommands for dealing with the following situations:

• user wants to add a new dependency zig fetch --save
• user wants to remove a dependency
• user wants to check for updates to dependency tree
• user wants to update one or more dependencies

A conflict occurs when dependency tree contains multiple projects with the same id, but different version.

Add conflict resolution logic. This could go one of two ways:

• The maximum version number is chosen but only among those projects actually referred to in the dependency tree by url/hash.
• User must explicitly specify how all conflicts are to be resolved whenever a conflict occurs. Conflict resolution data goes into build.zig.zon.
  • For each dependency on a project which has conflicts in the tree, user may choose that dependency to resolve to any project that exists within the dependency tree that has the same id, and a version that is greater or equal.