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

libexec/pyenv-virtualenv-realpath

@@ -0,0 +1,50 @@
+#!/usr/bin/env bash
+# Summary: Substitute realpath if unavailable as a builtin or file
+# Usage: . pyenv-virtualenv-realpath
+
+if ! {
+    enable -f "${BASH_SOURCE%/*}"/../libexec/pyenv-realpath.dylib realpath ||
+      type realpath
+  } >/dev/null 2>&1; then
+  if [ -n "$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 "$1"
+  }
+
+  realpath() {
+    local f="$*" \
+          name dir
+    [[ $f ]] || {
+      >&2 echo ${FUNCNAME[0]}: missing operand
+      return
+    }
+    while [[ -L $f ]]; do
+      f="$(resolve_link "$f")"
+    done
+    if [[ ! -d $f ]]; then
+      name="/${f##*/}"
+      # parent?
+      dir="${f%/*}"
+      if [[ $dir == $f ]]; then
+        #lacks /: parent is current directory
+        f="$PWD"
+      else
+        f="$dir"
+      fi
+    fi
+    #absolute directory
+    dir="$(cd "$f"
+           pwd)"
+    echo "$dir$name"
+  }
+fi