fix: pyenv-rehash | do NOT create conda_exclusion_list if conda is not installed.

I was doing some debugging with PYENV_DEBUG=1 and noticed that a lot of
work was being done in conda.bash, even though I had not installed any
conda versions like `mambaforge`.

The solution is pretty simple, put all the code inside an if-block.
This aligns with what @varikin found in his pull request #3037

Additionally, I have refactored the code slightly for readability
(created function `build_conda_exclusion_list`), and added comments
which should make it easier to understand what's going on.

This commit simplifies debugging and should reduce the execution time of

```bash
eval "$(pyenv init -)"
```

slightly.

pyenv.d/rehash/conda.bash

@@ -11,18 +11,34 @@ conda_exists() {
   [ -n "${condas}" ]
 }
 
+if conda_exists; then
+
+  # Reads the list of `blacklisted` conda binaries
+  # from `conda.d/default.list` and creates a function
+  # `conda_shim` to skip creating shims for those binaries.
+  build_conda_exclusion_list() {
     shims=()
     shopt -s nullglob
-for shim in $(cat "${BASH_SOURCE%/*}/conda.d/"*".list" | sort | uniq | sed -e 's/#.*$//' | sed -e '/^[[:space:]]*$/d'); do
+    for shim in $(cat "${BASH_SOURCE%/*}/conda.d/"*".list" | sort -u | sed -e 's/#.*$//' | sed -e '/^[[:space:]]*$/d'); do
       if [ -n "${shim##*/}" ]; then
         shims[${#shims[*]}]="${shim})return 0;;"
       fi
     done
     shopt -u nullglob
-eval "conda_shim(){ case \"\${1##*/}\" in ${shims[@]} *)return 1;;esac;}"
+    eval \
+"conda_shim() {
+  case \"\${1##*/}\" in
+    ${shims[@]}
+    *) return 1;;
+  esac
+}"
+  }
 
   # override `make_shims` to avoid conflict between pyenv-virtualenv's `envs.bash`
   # https://github.com/pyenv/pyenv-virtualenv/blob/v20160716/etc/pyenv.d/rehash/envs.bash
+  # The only difference between this `make_shims` and the `make_shims` defined
+  # in `libexec/pyenv-rehash` is that this one calls `conda_shim` to check
+  # if shim is blacklisted. If blacklisted -> skip creating shim.
   make_shims() {
     local file shim
     for file do
@@ -53,6 +69,6 @@ deregister_conda_shims() {
     fi
   }
 
-if conda_exists; then
+  build_conda_exclusion_list
   deregister_conda_shims
 fi