Rework filesystem mapping and partial overlap of --mapdir directories

[fschutt]

Currently when mapping directories in WASI with .map_dir(), the order of mapping is important, as /lib/api/blah could overwrite another mapping. Currently the order of mappings is important, but we should fix this in the next release or at least when we migrate / rework the [fs] mapping in the new Manifest format.

Also, duplicated mapping to / (root folder) aren't allowed, which makes dependency management impossible if two packages use the same folder name. (for example, having a dependency on python, which maps to / -> /usr/bin/python and a dependency on ruby mapping / -> /usr/bin/ruby). In the end /ruby.wasm and /python.wasm should both work, the only danger are conflicting file names (should be recognized and rejected when uploading).

[ptitSeb]

Related to #3476

[Michael-F-Bryan]

This is also related to #3678.

[syrusakbary]

Closing this in favor of #3678