makepkg: delete $srcdir upon clean build

Currently, the file glob used to clean the $srcdir misses dotfiles.

Instead remove the directory entirely and recreate it.

Almost all functions after the directory change are independent of $pwd, so don't change directory. $srcdir needs to exist though.
The only exception to this is extract_sources which depends on $pwd being $srcdir.

An alternative proposal (!88 (closed)) wanted to extend the file matching for deletion, but it was deemed impractical.

Closes !88 (closed) Closes #57 (closed)