Packaging with Poetry and other PEP-517 based packages

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I am attempting to package the pypi library [`pymedphys`](https://pypi.org/project/pymedphys/):

```toml
[[packaging_rule]]
type="pip-install-simple"
package="pymedphys"
```

When I run `pyoxidizer build`

This fails in the following way:
```
LookupError: https://files.pythonhosted.org/packages/20/e9/bae28bcfcb9942600b1e206d013499c291367ee8ddf9f455ff54729bc6e4/tomlkit-0.5.8.tar.gz#sha256=32c10cc16ded7e4101c79f269910658cc2a0be5913f1252121c3cd603051c269 (from https://pypi.org/simple/tomlkit/)
```

Out of interest when I try to build tomlkit I get the following:

```
LookupError: https://files.pythonhosted.org/packages/9d/6b/ad69758a595b6e0be4583d8ab298e43c4c0680278beceaa4a5e308f09a3c/clikit-0.4.0.tar.gz#sha256=6819a5b2a78523be485ea9e1ef380ae8d02da4bffed540f7881fdaafbff41390 (from https://pypi.org/simple/clikit/) (requires-python:>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*) is already being built: clikit<0.5.0,>=0.4.0 from https://files.pythonhosted.org/packages/9d/6b/ad69758a595b6e0be4583d8ab298e43c4c0680278beceaa4a5e308f09a3c/clikit-0.4.0.tar.gz#sha256=6819a5b2a78523be485ea9e1ef380ae8d02da4bffed540f7881fdaafbff41390 (from poetry>=0.12a3)
```

[pymedphys](https://github.com/pymedphys/pymedphys/blob/master/pyproject.toml#L166) and [tomlkit](https://github.com/sdispater/tomlkit/blob/master/pyproject.toml#L56) both use [Poetry](https://poetry.eustace.io/docs/pyproject/#poetry-and-pep-517) with its [PEP-517](https://www.python.org/dev/peps/pep-0517/) based build system. Is that potentially what is causing this issue?

Start by reproducing the shown packaging_rule for pymedphys and running `pyoxidizer build`. Inspect how PyOxidizer handles the Poetry and PEP-517 pyproject.toml files in pymedphys and tomlkit, then compare the dependency-resolution failures for tomlkit and clikit. Done means the affected packages build successfully or the unsupported packaging limitation is clearly identified.

Written by the indexing model from the issue text.