Introduce classpath_fragment.go.

A skeleton of a classpath fragment's interface and base to be used by
any modules that provide entries for *CLASSPATH variables at runtime.

In follow up changes, this would be extended to generate
packages/modules/SdkExtensions/proto/classpaths.proto configs for such
modules to be bundled into system and individual apex binaries.

Bug: 180105615
Test: m
Change-Id: I2df550862e97222c5650c4d0480c90231fd78ef0