uvloop fails to build (incorrect search path?)

I apologize in advance for how little information I'm about to give you, I'm very unfamiliar with Rust. I received this error when building a fresh project with just the following addition:

```
[[packaging_rule]]
type = "pip-install-simple"
package = "uvloop==0.11.2"
```

Here's a snippet of the error during `pyoxidizer build` ([the full output is here](https://gist.github.com/lithp/ec797b98c93e35c407f8523af3feb0b6)), there are a lot of undefined references

```
Compiling hello v0.1.0 (/tmp/tmp.12A0mm7S8g/hello)
error: linking with `cc` failed: exit code: 1
|
= [a very, very long command]
= note: /tmp/tmp.12A0mm7S8g/hello/build/target/x86_64-unknown-linux-gnu/debug/deps/libpyembed-6db96b47e95650bd.rlib(uvloop.loop.0.o): In function `__pyx_f_6uvloop_4loop_8UVHandle__close':
/tmp/pip-install-41hhka6_/uvloop/uvloop/loop.c:71955: undefined reference to `uv_close'
/tmp/pip-install-41hhka6_/uvloop/uvloop/loop.c:71955: undefined reference to `uv_close'
/tmp/tmp.12A0mm7S8g/hello/build/target/x86_64-unknown-linux-gnu/debug/deps/libpyembed-6db96b47e95650bd.rlib(uvloop.loop.0.o): In function `__pyx_f_6uvloop_4loop_14UVSocketHandle__fileno':
/tmp/pip-install-41hhka6_/uvloop/uvloop/loop.c:72392: undefined reference to `uv_fileno'
[a lot more lines like this, all complaining about undefined references]
```

This is despite the fact that I've installed `libuv1` and `libuv1-dev` (ubuntu packages which include headers and even a static library), and it occurs even when I pass `STATIC_DEPS=true`, [as suggested here](https://github.com/indygreg/PyOxidizer/issues/103#issuecomment-514878694).

I'm not sure what to try next, it seems like `pip` is building the library but cargo is never adding the library to the search path?

Start by reproducing the fresh project with the shown packaging_rule and uvloop==0.11.2, then inspect the full linker command in the linked build output. Check whether the libuv dependency is present in the generated build and linker search paths; done means pyoxidizer build completes without the undefined uv_close and uv_fileno references.

Written by the indexing model from the issue text.