Don't persist sourcemap changes between steps.

Currently, if 'eval-source-map' is enabled and you have multiple chunks with the same module in them, `EvalSourceMapDevToolModuleTemplatePlugin` runs its 'module' handler multiple times for the shared modules, and since it mutates the sourcemap, the mutations can be applied twice. This is mostly visible in 'sources', where it results in weird URLs. For example:

    webpack:///[resource-path]

becomes

    webpack:///webpack:///[resource-path]