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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