[build] Don't give drivers a SONAME

Drivers aren't libraries, so they don't need SONAMEs. The SONAME
libhid.so belongs to system/ulib/hid, not system/dev/input/hid.

Change-Id: If167608f639f54f69f4dde5ff95d77898ee09890

[devmgr][build] install/load drivers in/from /{boot,system}/driver

The legacy paths /boot/lib/driver and /system/lib/driver are still
supported for now, but will be removed once Fuchsia no longer needs
them.

Change-Id: Id270a9d515e87ba393d5d973fc484ebe66ccd2c8

[build] flatten the build

Previously we treated kernel/, system/, and third_party/ as
overlays on a shared namespace. This required the concept
of "canonical" module names, and a lot of complexity to ensure
that things didn't collide and the build worked.

This change gets rid of that, no longer passes -I to make,
so that include directives from our *.mk files do not magically
wildcard across various paths, etc.

The most user-visible change is that everywhere where a module
name is specified (MODULE_DEPS, MODULE_LIBS, etc), full module
names like kernel/lib/io or system/ulib/mxio must be used instead
of previously-allowed "short" names like lib/io and ulib/mxio.

The build output still has a similar shape, but the first segment
of the module path (kernel/, system/, or third_party/) is no
longer elided under $(BUILDDIR)

Change-Id: I525aba1da1c86eb7a86007bddc669f7eeebfedd5