This adds a Component type to the mozbuild.mozpack package manifest
parser, and teaches the packager to accept components of the form
[name destdir="dir"]. Then we update the Android package manifest and
simplify the packager code.
I would have liked to make the packager put mozglue.so and
MOZ_CHILD_PROCESS_NAME in lib/$(ABI_DIR) directly, but this turned out
to be awkward. Since MOZ_CHILD_PROCESS_NAME needs to have lib/ in its
name to load successfully on Android, we would have to add notation in
package manifests to install bin/lib/*plugin-container* to
lib/$(ABI_DIR)/*plugin-container*.
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