`--fetch` flag now has additional optional parameter, which specifies
how lazy dependencies should be fetched:
 * `needed` — lazy dependencies are fetched only if they are required
   for current build configuration to work. Default and works same
   as old `--fetch` flag.
 * `all` — lazy dependencies are always fetched. If `--system` flag
   is used after that, it's guaranteed that **any** build configuration
   will not require additional download of dependencies during build.
   Helpful for distro packagers and CI systems:
   https://www.github.com/ziglang/zig/issues/14597#issuecomment-1426827495

If none is passed, behaviour is same as if `needed` was passed.

Signed-off-by: Eric Joldasov <bratishkaerik@landless-city.net>