tools/manifestfile.py: Change library search to use a list of paths.

This commit changes how library packages are searched for when a manifest
file is loaded: there is now simply a list of library paths that is
searched in order for the given package.  This list defaults to the
main directories in micropython-lib, but can be added to -- either appended
or prepended -- by using `add_library()`.

In particular the way unix-ffi library packages are searched has changed,
because the `unix_ffi` argument to `require()` is now removed.  Instead, if
a build wants to include packages from micropython-lib/unix-ffi, then it
must explicitly add this to the list of paths to search using:

    add_library("unix-ffi", "$(MPY_LIB_DIR)/unix-ffi")

Work done in collaboration with Jim Mussared.

Signed-off-by: Damien George <damien@micropython.org>

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