Source the PKGBUILD using the correct path (relative to our base
directory) instead of using cd(1) to switch to the ABS base directory
first and to the package directory afterwards.

This is very useful when trying to track errors, also:

    $ ~/src/devtools/finddeps libdaq
    ./community/snort (depends)
    PKGBUILD: line 17: ruby: command not found
    PKGBUILD: line 19: [: =: unary operator expected

Versus:

    $ ~/src/devtools/finddeps libdaq
    ./community/snort (depends)
    ./community/ruby-pkgconfig/PKGBUILD: line 17: ruby: command not found
    ./community/lmms/PKGBUILD: line 19: [: =: unary operator expected

Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>