Remove "extends" in `incremental_webpack_compiler` and add "_" to privates
What does this MR do?
extends
represents an "is-a" relationship. It does not make sense to say that an IncrementalWebpackCompiler
is a NoopCompiler
. These two classes have the same interface, they don't need to extend eachother
While we're here, why not also use the leading _
convention to specify privates?