create_venv should use symlinks instead of hard copies

[baxelrod-bdai]

When you use create_venv, the files in <venv>/lib/python3.10/site-packages are all physical copies of the dependencies. It would be much better if they were simply symlinks to the Bazel cache and source code. This makes it so that if you use F12 to "Go to definition" in an IDE, you can modify the actual code.

rules_pyvenv works this way:

pip dependencies point to the Bazel cache:

baxelrod@baxelrod-ThinkPad:~/repos/rai-bazel-sandbox/.venv/lib/python3.10/site-packages$ ll termcolor/termcolor.py 
lrwxrwxrwx 1 baxelrod baxelrod 158 Aug 27 10:49 termcolor/termcolor.py -> /home/baxelrod/.cache/bazel/_bazel_baxelrod/223f490fa0c2f2ec38707ddac3dd6272/external/rules_python++pip+pip_310_termcolor/site-packages/termcolor/termcolor.py

local targets point to the source code:

baxelrod@baxelrod-ThinkPad:~/repos/rai-bazel-sandbox/.venv/lib/python3.10/site-packages$ ll hello_lib.py 
lrwxrwxrwx 1 baxelrod baxelrod 61 Aug 27 10:49 hello_lib.py -> /home/baxelrod/repos/rai-bazel-sandbox/hello_lib/hello_lib.py*