Support incremental builds

[harendra-kumar]

packcheck creates a source distribution in .packcheck directory and builds from that. To reuse the previous build's artifacts we can do the following:

• Create the dist-newstyle directory outside the source distribution we build from
• Remove the source distribution on a rebuild, do not remove the dist-newstyle dir
• When we untar the source distribution, restore the exact timestamps
• Use dist-newstyle dir name based on a sha256 hash of the build parameters so that we can use different build dirs for different builds in the build matrix.