Concurrent builds of watched files in the same cache group can cause corruption

[skeggse]

This is due to how we modify the cached modules in the post-build callback.

Error: Could not load <redacted remote import> (imported by <redacted root shim>): ENOENT: no such file or directory, open '<redacted remote import>'
    at .../node_modules/rollup/dist/rollup.js:21385:19
    at <anonymous>
    at process._tickDomainCallback (internal/process/next_tick.js:228:7)
Error: Could not load underscore (imported by <redacted>): ENOENT: no such file or directory, open 'underscore'
    at .../node_modules/rollup/dist/rollup.js:21385:19
    at <anonymous>
Error: Could not load backbone (imported by <redacted>): ENOENT: no such file or directory, open 'backbone'
    at .../node_modules/rollup/dist/rollup.js:21385:19
    at <anonymous>

cc @wearhere