PyInstaller, how to include data files from an external package that was installed by pip?

I solved this by taking advantage of the fact that the spec file is Python code that gets executed. You can get the root of the package dynamically during the PyInstaller build phase and use that value in the datas list. In my case I have something like this in my .spec file:

import os
import importlib

package_imports = [['package_name', ['file0', 'file1']]

datas = []
for package, files in package_imports:
    proot = os.path.dirname(importlib.import_module(package).__file__)
    datas.extend((os.path.join(proot, f), package) for f in files)

And use the the resulting datas list as a parameters to Analysis.

Here's a one-liner using the same idea as Turn mentioned. In my case I needed a package (zbarcam) that was inside of kivy_garden. But I tried to generalize the process here.

from os.path import join, dirname, abspath, split
from os import sep
import glob
import <package>

pkg_dir = split(<package>.__file__)[0]
pkg_data = []
pkg_data.extend((file, dirname(file).split("site-packages")[1]) for file in glob.iglob(join(pkg_dir,"**{}*".format(sep)), recursive=True))