fix(py_venv): Activate embedded interpreters (#576)

Previously the `bin/activate` script relied on Bazel (specifically
`bazel run`) defined envvars, or being sourced directly from the binary
entrypoint. This meant that if the user created a link (eg via the venv
link verb or manually) to the venv defined by a given target, sourcing
that venv's activate script would fail to locate a runfiles-based
interpreter. On systems where there is no appropriately versioned
non-hermetic interpreter available, this meant that venvs which should
run with the Bazel-managed interpreter instead failed.

The fix is to harden venv initialization so that if the `activate`
script is directly used AND the runfiles preconditions are not met an
effort will be made to satisfy them.

---

### Changes are visible to end-users: yes

- Searched for relevant documentation and updated as needed: no
- Breaking change (forces users to change their own code or config): no
- Suggested release notes appear below: no

### Test plan

- [x] Updated script works on BASH
```shellsession
$ /bin/bash
$ bazel run //examples/py_binary:py_binary.venv
INFO: Analyzed target //examples/py_binary:py_binary.venv (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
Target //examples/py_binary:py_binary.venv up-to-date:
  bazel-bin/examples/py_binary/py_binary.venv
INFO: Elapsed time: 1.224s, Critical Path: 0.00s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action
INFO: Running command line: bazel-bin/examples/py_binary/py_binary.venv
Linking: /private/var/tmp/_bazel_arrdem/93bfea6cdc1153cc29a75400cd38823a/execroot/aspect_rules_py/bazel-out/darwin_arm64-fastbuild/bin/examples/py_binary/.py_binary.venv -> /Users/arrdem/Documents/work/aspect/rules_py/.py_binary.venv

To activate the virtualenv run:
    source /Users/arrdem/Documents/work/aspect/rules_py/.py_binary.venv/bin/activate

Link is up to date!
$ ( source .py_binary.venv/bin/activate; echo $VIRTUAL_ENV; which python; which python3; echo $PYTHONHOME; deactivate; echo $VIRTUAL_ENV; which python; which python3 )
/private/var/tmp/_bazel_arrdem/93bfea6cdc1153cc29a75400cd38823a/execroot/aspect_rules_py/bazel-out/darwin_arm64-fastbuild/bin/examples/py_binary/.py_binary.venv
/private/var/tmp/_bazel_arrdem/93bfea6cdc1153cc29a75400cd38823a/execroot/aspect_rules_py/bazel-out/darwin_arm64-fastbuild/bin/examples/py_binary/.py_binary.venv/bin/python
/private/var/tmp/_bazel_arrdem/93bfea6cdc1153cc29a75400cd38823a/execroot/aspect_rules_py/bazel-out/darwin_arm64-fastbuild/bin/examples/py_binary/.py_binary.venv/bin/python3
/private/var/tmp/_bazel_arrdem/93bfea6cdc1153cc29a75400cd38823a/external/python_toolchain_aarch64-apple-darwin

/Users/arrdem/.local/pyenv/shims/python
/Users/arrdem/.local/pyenv/shims/python3
```

- [x] Updated script works on ZSH
```shellsession
$ /bin/zsh
$ bazel run //examples/py_binary:py_binary.venv
INFO: Analyzed target //examples/py_binary:py_binary.venv (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
Target //examples/py_binary:py_binary.venv up-to-date:
  bazel-bin/examples/py_binary/py_binary.venv
INFO: Elapsed time: 1.224s, Critical Path: 0.00s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action
INFO: Running command line: bazel-bin/examples/py_binary/py_binary.venv
Linking: /private/var/tmp/_bazel_arrdem/93bfea6cdc1153cc29a75400cd38823a/execroot/aspect_rules_py/bazel-out/darwin_arm64-fastbuild/bin/examples/py_binary/.py_binary.venv -> /Users/arrdem/Documents/work/aspect/rules_py/.py_binary.venv

To activate the virtualenv run:
    source /Users/arrdem/Documents/work/aspect/rules_py/.py_binary.venv/bin/activate

Link is up to date!
$ ( source .py_binary.venv/bin/activate; echo $VIRTUAL_ENV; which python; which python3; echo $PYTHONHOME; deactivate; echo $VIRTUAL_ENV; which python; which python3 )
/private/var/tmp/_bazel_arrdem/93bfea6cdc1153cc29a75400cd38823a/execroot/aspect_rules_py/bazel-out/darwin_arm64-fastbuild/bin/examples/py_binary/.py_binary.venv
/private/var/tmp/_bazel_arrdem/93bfea6cdc1153cc29a75400cd38823a/execroot/aspect_rules_py/bazel-out/darwin_arm64-fastbuild/bin/examples/py_binary/.py_binary.venv/bin/python
/private/var/tmp/_bazel_arrdem/93bfea6cdc1153cc29a75400cd38823a/execroot/aspect_rules_py/bazel-out/darwin_arm64-fastbuild/bin/examples/py_binary/.py_binary.venv/bin/python3
/private/var/tmp/_bazel_arrdem/93bfea6cdc1153cc29a75400cd38823a/external/python_toolchain_aarch64-apple-darwin

/Users/arrdem/.local/pyenv/shims/python
/Users/arrdem/.local/pyenv/shims/python3
```

Author: arrdem

docs/venv.md

@@ -10,6 +10,13 @@ exports_files([
     "link.py",
 ])
 
+config_setting(
+    name = "debug_build",
+    values = {
+        "compilation_mode": "dbg",
+    },
+)
+
 bzl_library(
     name = "py_venv",
     srcs = [

py/private/py_venv/BUILD.bazel

@@ -1,15 +1,15 @@
 #!/usr/bin/env bash
 
+if {{DEBUG}}; then
+    set -x
+fi
+
 {{BASH_RLOCATION_FN}}
 
 runfiles_export_envvars
 
 set -o errexit -o nounset -o pipefail
 
-{{PRELUDE}}
-
 source "$(rlocation "{{VENV}}")"/bin/activate
 
-{{PREEXEC}}
-
 exec "$(rlocation "{{VENV}}")"/bin/python {{INTERPRETER_FLAGS}} "$@"