The less-imports.js utility broke on LessCSS 3.11.2, and again on

3.11.3, but in different ways.
On prior versions, the internal list of imported files was an object,

mapping file paths to some internal state. This was removed entirely on

3.11.2, leaving no way to get to these files (there are other mappings

that include filenames, but they also include the original file being

processed).
3.11.3 brought this back, but changed it to an array. That caused the

existing logic to fail, as we were calling Object.keys() on the list,

giving us dependencies that were indexes into an array. This in turn

caused every file to appear out of date, every time it was loaded,

explaining the recent slowdown in LessCSS file processing.
We now check if files exists at all, and bail with a slightly more

helpful error that at least leads us to knowing there's a problem.

Otherwise, we proceed to handle the list correctly, whether it's an

object or an array.

Tested an install that ran on 3.11.1, and an install on 3.11.3.

Verified that the dependencies for files were loaded correctly on

each version.