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

bin/pyenv-sh-activate

@@ -177,7 +177,8 @@ EOS
 esac
 
 # anaconda/miniconda
-if [ -x "${prefix}/bin/conda" ]; then
+if [ -d "${prefix}/conda-meta" ] ||
+     [ -x "${prefix}/bin/conda" ]; then
   if [[ "${prefix}" != "${prefix%/envs/*}" ]]; then
     CONDA_DEFAULT_ENV="${venv##*/envs/}"
   else
@@ -233,7 +234,8 @@ EOS
 fi
 
 # conda package anaconda/miniconda scripts (#173)
-if [ -x "${prefix}/bin/conda" ]; then
+if [ -d "${prefix}/conda-meta" ] ||
+     [ -x "${prefix}/bin/conda" ]; then
   shopt -s nullglob
   case "${shell}" in
   fish )