Local package pool now implements more generic package pool API.
The base idea is to never expose full paths to files, so that other
kinds of package pools (e.g. package pool in S3) could be used to implement
the same interface.
Files get into the pool only using `Import` method. `Import` method is
now more smart, it supports moving files into the pool, it can detect if
files reside on the same filesystem and use hardlinking instead of copying.
This will make direct mirror downloads still as fast as they were with previous
version which was performing download directly to package pool.
New package pool doesn't have two things implemented yet:
1. New file placement according to SHA256 or other configured hash
2. Calculate at least SHA256/MD5 for each imported files.
MD5 would be required for S3/Swift publishing
This is related to #506
As a first step, don't pass MD5 explicitly, pass checksum info object,
so that as a next step we can choose which hash to use.
There should be no functional changes so far.
Next step: stop returning explicit paths from public package pool.
Andrey Smirnov
Clemens Rabe
Chris Read