packages/pkgbuild: internal makepkg: wrap main execution path into function.

This will allow to pre-source the file in the main shell and only call
the init function in the sub-shell, speeding up execution.
packages/pkgbuild: pre-source internal makepkg to speed up execution.
packages/pkgbuild: pkgbuild_sources: add option for reporting all source files, incl. fetched ones.

Also now source files (and directories) are checked for existence to
handle non-empty SRCDEST (however, these files in SRCDEST are ignored
completely).