Fix fetchMavenArtifact to forward the "meta" attribute to mkDerivation rather than fetchurl.

By fixing that issue, it adds support for the "meta" attribute of fetchMavenArtifact, making it usable as direct replacement for stdenv.mkDerivation in most cases.

Background: For many Java packages, the derivation created by fetchMavenArtifact is wrapped into another derivation that copies over all files, just to be able to specify the metadata via the "meta" attribute in the outer derivation. This is cumbersome, because it is extra code that is unnecessarily convoluted. And it is wasteful, as all downloaded JAR files are stored twice into the Nix store, for the outer and the inner derivation, until the next "gc" or "optimize" action happens.

Solution: All those derivations could instead be created by a single call to fetchMavenArtifact, without copying their downloaded JAR files twice into the Nix store, if fetchMavenArtifact would just forward the "meta" attribute properly. This commit does exactly that.

See PR #427538 for an example on how simple Java derivations from Maven Artifacts can be:

{ fetchMavenArtifact, lib }:
fetchMavenArtifact {
  groupId = "...";
  artifactId = "...";
  version = "...";
  hash = "sha256-...";
  meta = {
    homepage = "....";
    description = "...";
    license = ...;
    sourceProvenance = [ lib.sourceTypes.binaryBytecode ];
    maintainers = [ ... ];
  };
}

Also, reusing pieces of another package, such as its version number or most of it metadata, works the exact same way as we are used to with stdenv.mkDerivation:

{ fetchMavenArtifact, lib, some-other-package }:
fetchMavenArtifact {
  groupId = "...";
  artifactId = "...";
  inherit (some-other-package) version;
  hash = "sha256-...";
  meta = some-other-package.meta // {
    description = "...";
    sourceProvenance = [ lib.sourceTypes.binaryNativeCode ];
  };
}

Again, see PR #427538 for a practical example of reusing metadata.

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
  • (via the junixsocket-* packages, see PR junixsocket{,-native}-common: init at 2.10.1 #427538)
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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