Fix pre-releases causing corrective changes

The way we get all the available versions returns also the pre-releases.
But unless a `--pre` flag is added to $install_args, we don't want to
update to them when $ensure set to 'latest'.

manifests/pip.pp

@@ -238,19 +238,25 @@
       }
 
       'latest': {
+        $include_pre_releases = $install_args =~ /(^| )--pre($| )/
+
+        $exclude_pre_releases_command = $include_pre_releases ? {
+            true    => '',
+            default => ' | grep -vP "(a|b|c|rc|alpha|beta|pre|preview)[-_\.]?(\d+)?"',
+        }
+
         # Unfortunately this is the smartest way of getting the latest available package version with pip as of now
-        # Note: we DO need to repeat ourselves with "from version" in both grep and sed as on some systems pip returns
-        # more than one line with paretheses.
-        $latest_version = join(["${pip_env} install ${proxy_flag} ${pkgname}==notreallyaversion 2>&1",
-                                ' | grep -oP "\(from versions: .*\)" | sed -E "s/\(from versions: (.*?, )*(.*)\)/\2/g"',
-                                ' | tr -d "[:space:]"'])
+        $latest_version = "${pip_env} install ${pypi_index} ${proxy_flag} ${pkgname}==notreallyaversion 2>&1 \
+                          | grep -oP '\(from versions: \K(.*)(?=\))' | tr ',' '\n' \
+                          $exclude_pre_releases_command \
+                          | tail -n1"
 
         # Packages with underscores in their names are listed with dashes in their place in `pip freeze` output
         $pkgname_with_dashes = regsubst($pkgname, '_', '-', 'G')
         $grep_regex_pkgname_with_dashes = "^${pkgname_with_dashes}=="
-        $installed_version = join(["${pip_env} freeze --all",
-                                  " | grep -i -e ${grep_regex_pkgname_with_dashes} | cut -d= -f3",
-                                  " | tr -d '[:space:]'"])
+        $installed_version = "${pip_env} freeze --all \
+                             | grep -i -e ${grep_regex_pkgname_with_dashes} | cut -d= -f3 \
+                             | tr -d '[:space:]'"
 
         $unless_command = "[ \$(${latest_version}) = \$(${installed_version}) ]"