title: found a virtual manifest at <path> instead of a package manifest
body: <p>I searched for <code>[rust] "instead of a package manifest"</code> on this site before asking and found no hits. I also read about virtual manifests <a href="https://doc.rust-lang.org/cargo/reference/workspaces.html" rel="noreferrer">here</a> but did not resolve my question.</p>
  
  <p>My goal is to make changes to <a href="https://github.com/maps4print/azul" rel="noreferrer">azul</a>.</p>
  
  <p>To achieve this I read about patching dependencies <a href="https://doc.rust-lang.org/cargo/reference/overriding-dependencies.html#overriding-repository-url" rel="noreferrer">here</a>, and now I have this <code>Cargo.toml</code></p>
  
  <pre><code>[package]
  name = "my_first_azul_app"
  version = "0.1.0"
  authors = ["Name &lt;Email&gt;"]
  edition = "2018"
  
  # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
  
  [dependencies]
  azul = { git = "https://github.com/maps4print/azul" }
  
  [patch."https://github.com/maps4print/azul"]
  azul = { path = "../azul" }
  </code></pre>
  
  <p>In path <code>../azul</code> I have checked out the azul project with <code>git clone</code>. In <code>main.rs</code> I have followed <a href="https://github.com/maps4print/azul/wiki/Installation#adding-azul-to-your-dependencies" rel="noreferrer">this</a> to get,</p>
  
  <pre><code>extern crate azul;
  
  fn main() {
      println!("Hello world!");
  }
  </code></pre>
  
  <p>Then I try to test</p>
  
  <pre><code>$ cargo run
  error: failed to resolve patches for `https://github.com/maps4print/azul`
  
  Caused by:
    failed to load source for a dependency on `azul`
  
  Caused by:
    Unable to update /home/name/projects/azul
  
  Caused by:
    found a virtual manifest at `/home/name/projects/azul/Cargo.toml` instead of a package manifest
  </code></pre>
  
  <p>I do not understand the final caused by line.
  If I remove the <code>[patch]</code> configuration, it "works".
  Quoting because it fails to compile, but that is why I am trying to check it out and attempt a fix. What charges do I need to make to develop the <code>azul</code> dependency?</p>
  
  <p>TIA,</p>