Add 'debug' option which preserves failed package dirs

[fingolfin]

If a package fails to install, it is normally deleted to prevent stale
packages from clogging up the system. But for debugging the failure it
is useful to keep the package around.

[codecov]

Codecov Report

Merging #119 (c868686) into master (4d8e91f) will increase coverage by 0.00%.
The diff coverage is 83.33%.

@@           Coverage Diff           @@
##           master     #119   +/-   ##
=======================================
  Coverage   83.23%   83.23%           
=======================================
  Files           4        4           
  Lines        1020     1032   +12     
=======================================
+ Hits          849      859   +10     
- Misses        171      173    +2     

Files Changed	Coverage Δ
gap/PackageManager.gi	81.91% <83.33%> (+0.01%)	⬆️

[james-d-mitchell]

I think this is a great idea!

[fingolfin]

Note that I did not add the check to the PKGMAN_RemoveDir call in RemovePackage because that's a core part of the functionality of RemovePackage -- we really want it to remove the package.