This introduces a new djblets.pipeline.compilers.mixins module, which

currently contains SourceMapStaleCheckMixin. This new mixin allows

Pipeline compilers to efficiently check whether a file in a bundle needs

to be rebuilt, based on any files that contributed to its sourcemap.
This works by looking up all the source files in the sourcemap and

comparing timestamps. If any timestamps are newer than that of the built

file or sourcemap, or if any files are missing, then the built file is

considered stale and needs to be rebuilt.
The list of files are extracted from the sourcemap and cached for a

period of 1 hour, to avoid having to re-parse the sourcemap every time.
This will be used for the upcoming rollup.js support for bundling

module-based JavaScript files. Unfortunately, it can't currently be used

for LessCSS due to a broken sourcemap implementation that prevents some

files from being included, but that may be tackled separately via a new

plugin or build step.

Tested this thoroughly with the upcoming rollup.js support.