Auto detect workspace_member for git sources.

Cargo does this for git repositories as well:

rust-lang/cargo#1462 (comment)

This will fix #33, #53 -- integration tests pending.

It depends on a nixpkgs buildRustCrate feature PR:

NixOS/nixpkgs#82155

crate2nix/templates/build.nix.tera

@@ -105,6 +105,7 @@ rec {
         {%- elif crate.source.LocalDirectory.path %}
         src = (builtins.filterSource sourceFilter {{crate.source.LocalDirectory.path | safe}});
         {%- elif crate.source.Git %}
+        workspace_member = null;
         src = pkgs.fetchgit {
           url = {{crate.source.Git.url}};
           rev = {{crate.source.Git.rev}};

sample_projects/bin_with_git_branch_dep/Cargo.nix

@@ -98,6 +98,7 @@ rec {
         crateName = "nix-base32";
         version = "0.1.2-alpha.0";
         edition = "2018";
+        workspace_member = null;
         src = pkgs.fetchgit {
           url = "https://github.com/kolloch/nix-base32";
           rev = "42f5544e51187f0c7535d453fcffb4b524c99eb2";

sample_projects/bin_with_git_submodule_dep/Cargo.nix

@@ -736,6 +736,7 @@ rec {
         crateName = "librocksdb-sys";
         version = "6.2.4";
         edition = "2015";
+        workspace_member = null;
         src = pkgs.fetchgit {
           url = "https://github.com/rust-rocksdb/rust-rocksdb";
           rev = "64bd09899306d23dfd1504465bd1e9adb15fb9ca";
@@ -980,6 +981,7 @@ rec {
         crateName = "rocksdb";
         version = "0.13.0";
         edition = "2015";
+        workspace_member = null;
         src = pkgs.fetchgit {
           url = "https://github.com/rust-rocksdb/rust-rocksdb";
           rev = "64bd09899306d23dfd1504465bd1e9adb15fb9ca";