support newer conda

pyenv-virtualenvs could not list conda environments & pyenv shell would only activate the base conda environment
the conda detection criteria of testing the presence of `conda` or `activate` files under `$(pyenv root)/versions/$version/bin` appears to be the culprit, since newer environments no longer include these files: those files reside in the base conda environment
- add detection criteria of `$(pyenv root)/versions/$version/conda-meta`
- compute the real prefix to the base environment from `realpath $(realpath $(pyenv root)/versions/$version)/../..`
- to allow that, enhance substitute `realpath` in `pyenv-virtualenvs` to reduce relative paths `.` & `..`, and factor that code out to a file under `libexec` for reuse
- hook `which` to locate conda from the real prefix

etc/pyenv.d/which/conda.bash

@@ -0,0 +1,11 @@
+# newer versions of conda share programs from the real prefix
+# this hook tries to find the executable there
+
+if [ ! -x "${PYENV_COMMAND_PATH}" ] && [[ "${PYENV_COMMAND_PATH##*/}" == "conda" ]]; then
+  if [ -d "${PYENV_ROOT}/versions/${version}/conda-meta" ]; then
+    conda_command_path="$(pyenv-virtualenv-prefix "$version")"/bin/"${PYENV_COMMAND_PATH##*/}"
+    if [ -x "${conda_command_path}" ]; then
+      PYENV_COMMAND_PATH="${conda_command_path}"
+    fi
+  fi
+fi