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-virtualenvs

@@ -7,6 +7,7 @@
 
 set -e
 [ -n "$PYENV_DEBUG" ] && set -x
+. "${BASH_SOURCE%/*}"/../libexec/pyenv-virtualenv-realpath
 
 if [ -z "$PYENV_ROOT" ]; then
   PYENV_ROOT="${HOME}/.pyenv"
@@ -32,38 +33,6 @@ done
 
 versions_dir="${PYENV_ROOT}/versions"
 
-if ! enable -f "${BASH_SOURCE%/*}"/../libexec/pyenv-realpath.dylib realpath 2>/dev/null; then
-  if [ -n "$PYENV_NATIVE_EXT" ]; then
-    echo "pyenv: failed to load \`realpath' builtin" >&2
-    exit 1
-  fi
-
-  READLINK=$(type -p greadlink readlink | head -1)
-  if [ -z "$READLINK" ]; then
-    echo "pyenv: cannot find readlink - are you missing GNU coreutils?" >&2
-    exit 1
-  fi
-
-  resolve_link() {
-    $READLINK "$1"
-  }
-
-  realpath() {
-    local cwd="$PWD"
-    local path="$1"
-    local name
-
-    while [ -n "$path" ]; do
-      name="${path##*/}"
-      [ "$name" = "$path" ] || cd "${path%/*}"
-      path="$(resolve_link "$name" || true)"
-    done
-
-    echo "${PWD}/$name"
-    cd "$cwd"
-  }
-fi
-
 if [ -d "$versions_dir" ]; then
   versions_dir="$(realpath "$versions_dir")"
 fi