hghave: list the module needed for the `vcr` check

I'm tired of having to look up modules each time I setup a system, and try to
distinguish between similar package names to get the right one.  Now that the
search API has been disabled, it's even harder.  There are other python packages
here that should be listed like this, but this is the one that came up missing
today, so it's a start.

Differential Revision: https://phab.mercurial-scm.org/D9879

tests/hghave.py

@@ -1034,7 +1034,7 b' def has_sqlite():'
     return matchoutput('sqlite3 -version', br'^3\.\d+')
 
 
-@check('vcr', 'vcr http mocking library')
+@check('vcr', 'vcr http mocking library (pytest-vcr)')
 def has_vcr():
     try:
         import vcr