setup: problems with local install

[Ken Brown]

This is a followup to the thread started here:

   https://cygwin.com/ml/cygwin/2018-03/msg00027.html

There are two problems with installing from a local directory.

1. In the Category view, "No packages found" is displayed where it 
should say "All" in the first line of the list.  This is probably just a 
cosmetic issue, and I haven't tried to track it down yet.

2. In several of the views, all packages from setup.ini are listed, even 
if there is no corresponding archive in the local directory.  What 
happens is that packagemeta::scan() calls pkg.source()->sites.clear() 
for such packages, but this information is never used to prevent the 
package from appearing in the list.

It used to be that such packages would be declared inaccessible, but 
SolvableVersion::accessible() no longer does this.

Jon, you wrote the following comment in the definition of 
SolvableVersion::accessible():

"The 'accessible' check used to test if an archive was available locally 
or from a mirror.  This seems utterly pointless as binary packages which 
aren't 'accessible' never get to appear in the package list."

Do we need to reinstate the old function of the accessibility check?

Ken