(maint) replace legacy validate function with datatype

This replaces the validate_absolute_path() function call with a datatype
from puppetlabs/stdlib. The type is available since the 4.13.1 release
of stdlib (and this module already depends on 5.1 and newer).

Some background: I'm currently debugging a slow pe-puppetserver metrics
API. The puppet-profiler is enabled and provides metrics for all called
functions. In our environment the puppet_agent module is the only one
still using this legacy function. By replacing it with a datatype, it
follows current best practice guides and also speeds up the API because
it has to care about less metrics (in my specific case at least).

Author: bastelfreak

manifests/init.pp

@@ -117,7 +117,7 @@
   $aix_source              = 'puppet:///pe_packages',
   $use_alternate_sources   = false,
   $alternate_pe_source     = undef,
-  $install_dir             = undef,
+  Optional[Stdlib::Absolutepath] $install_dir = undef,
   $disable_proxy           = false,
   $proxy                   = undef,
   $install_options         = [],
@@ -146,10 +146,6 @@
     fail('Only one of $source and $absolute_source can be set')
   }
 
-  if $facts['os']['family'] == 'windows' and $install_dir != undef {
-    validate_absolute_path($install_dir)
-  }
-
   if $package_version == undef {
     info('puppet_agent performs no actions if a package_version is not specified')
   } elsif defined('$::pe_server_version') {