[python] suggest correct package name when lxml is missing

Summary:
The error was complaining about a missing `etree` package but that's actually
part of the `lxml` package and that wasn't clear from the output.

See for instance https://github.com/facebook/infer/issues/635

Reviewed By: martinoluca

Differential Revision: D4905953

fbshipit-source-id: 28b26af

infer/lib/python/inferlib/issues.py

@@ -201,7 +201,7 @@ def merge_reports_from_paths(report_paths):
 
 def _pmd_xml_of_issues(issues):
     if etree is None:
-        print('ERROR: "etree" Python package not found.')
+        print('ERROR: "lxml" Python package not found.')
         print('ERROR: You need to install it to use Infer with --pmd-xml')
         sys.exit(1)
     root = etree.Element('pmd')