This adds a `add_library(name, path)` method for use in manifest.py that
allows registering an external path (e.g. to another repo) by name.
This name can then be passed to `require("package", library="name")` to
reference packages in that repo/library rather than micropython-lib.
Within the external library, `require()` continues to work as normal
(referencing micropython-lib) by default, but they can also specify the
library name to require another package from that repo/library.
Signed-off-by: Jim Mussared <jim.mussared@gmail.com>
5d28bb4adb