The first URL from PyPI is often a platform-specific wheel, so it's
better if we prefer to grab a source distribution that we can build with
emscripten.
Before:
```
$ bin/pyodide mkpkg lxml
$ grep 'url: ' packages/lxml/meta.yaml
url: 951337aa3d28589eccca765c4e17ce/lxml-4.3.2-cp27-cp27m-macosx_10_6_intel.macosx_10_9_intel.macosx_10_9_x86_64.macosx_10_10_intel.macosx_10_10_x86_64.whl
```
After:
```
$ bin/pyodide mkpkg lxml
$ grep 'url: ' packages/lxml/meta.yaml
url: d070609b410dbf4233659af53fe0ab/lxml-4.3.2.tar.gz
```
Closes: #363.