Products as objects; replace string representation of Products with objects

[ity]

From email conversations:

We want to use opaque (they extend object - no particular relationship until we see a need) types, ie: CompileClasspath, RuntimeClasspath, ClassfileMapping, MarkdownMapping, etc. Each of which is a type exported by a backend. The implication is that if, for example, you write a new jvm task that needs the RuntimeClasspath, you'll need a BUILD/import dep on the pants-provided jvm backend.

[stuhood]

Stale... more recent discussion on #3560