Enforce correct rust library file names.

rustc expects libraries and proc_macro filenames to conform to
a particular format, alphanumeric with underscores and lib${crate_name}.*.
Enforce this with a check when getStem() is called.

This makes the crate_name property required for proc_macros and
libraries. This also removes the notion of a default crate name derived
from the module name. It's not needed for binaries, so this won't impact
them.

Bug: 143579265
Test: m -j crosvm.experimental
Change-Id: I2770cf7d02dd4291c3d240d58d242b940098dcee