packages/pkgbuild: pkgbuild_need_update: force-update packages without Git repo initialized.
packages/pkgbuild: pkgbuild_need_update: query also official repos for a package version.
packages/pkgbuild: _pkgbuild_update_selective_revert: sanitize calling convention: assume in git working tree.
packages/pkgbuild: _pkgbuild_update_selective_revert: sanitize calling convention: assume in package's build tree.
packages/pkgbuild: pkgbuild_{version,is_devel,devel_suffix}: move to beginning of file and comment nonstandard calling convention.

They take a PKGBUILD file, whereas we now convert all functions to take build directory.
Mark them accordingly.
packages/pkgbuild: pkgbuild_name: add another interrogatory function for getting package name by PKGBUILD.
packages/pkgbuild: pkgbuild_need_update: make a bit more `set -e`-safe.
packages/pkgbuild: sanitize calling convention of all functions; rewrite pkgbuild_{need_,}update.

Calling convention: all functions, except interrogatory (specially commented),
take a path to the build directory, not path to PKGBUILD.

Rewrite: the case of existing, but not initialized, build directory is now handled
gracefully.
Also, nothing now relies on naming build directories after packages.