Support extracting language versions from files

[mjpitz]

While the extractor pulls out dependency versions, it doesn't pull out language versions.

Java - pom - https://www.baeldung.com/maven-java-version
Java - gradle - https://docs.gradle.org/current/userguide/building_java_projects.html
NodeJS - package.json - https://docs.npmjs.com/cli/v6/configuring-npm/package-json#engines
Go - go.mod - go key
Go - Godeps - https://github.com/tools/godep/blob/master/Godeps/Godeps.json#L3
Rust - Cargo.toml - not set?
Composer - https://andy-carter.com/blog/composer-php-platform

This shouldn't be too hard since we're already in there.

[mjpitz]

This should be stored on the edge between the source and module.

[mjpitz]

This will require an associated depscloud/api change