base: Search for 'meson.build' in the right path

Commit 2cc433db naively tested for the cwd, assuming that jhbuild buildone would switch you to the checkout path of the automatic module, so the condition was never met unless your shell's cwd was the checked out module. Now it builds the expected path of checkoutdir + modname.

This also inverts the if-else so that we search for autogen.sh instead of meson.build. This means that when the code has not been checked out yet (so that there's no files at all) this will default to meson.