packages/pkgbuild: fix new-style SVN repo handling.

SVN repos are extracted using "svn export", which strips the copy of all
SVN-related data, so it becomes impossible to run "svn info" on the
extracted copy (what we attempt to do).

Instead, make an exception for SVN to gather information from the source
root, not extracted one.

Legacy repositories always reside in $srcdir, which is now also clearly
stated in code.
packages/pkgbuild: _pkgbuild_vcs_process_svn: handle fragments.

This function works on source root, so "svn info" shall get "-r"
parameter if there is a specific revision to extract.

We have no way here but to mimic makepkg logic in order to make data
extraction (makepkg:extract_svn()) coherent with our timestamp
extraction.
packags/pkgbuild: add _pkgbuild_vcs_has_legacy, copycat of _pkgbuild_vcs_handle_legacy.

The new function just checks presence of legacy VCS sources.
packages/pkgbuild: pkgbuild_sanitize: add new VCS support.

If a package is detected to use new VCS system, CLEAN_BUILD makes
sanitizer to remove src/ and not src/*build*/.