Handle file conflicts smarter when a file moves between packages

[joebonrichie]

The issue:
pkg B v2 is attempting to install /usr/bin/foo but /usr/bin/foo already exists on
the system from pkg A v1

• pkg A v2 also is scheduled to install and no longer contains /usr/bin/foo but pkg B
  v2 is scheduled ahead of it.

Currently the upgrade operations completely ignores file conflicts and the install operation has a ugly workaround for this edge case. This edge case can be surprisingly common for example functionality getting split off into it's own library.

PiSi has no knowledge of the file lists for remote pkgs and adding support for that is outside of this scope. However, we can

If we encounter a file conflicts when attempting to install a package

• Check if the existing conflicting package also exists in the install order
• If it doesn't also exist in the install order, abort
• If it does, extract and read the files.xml file to see if the conflicting file(s) still exists in the latest version of the package (the package should already be downloaded at this point)
• If it doesn't allow installation to continue but warn user of a file(s) moving from pkg A to pkg B
• If it does abort