We have a specialization of Pipeline's LessCompiler that looks for all

dependencies recursively in a LessCSS file, making use of an in-house

script. If any dependency changes, we consider the file we want to

compile to be outdated.
This operation can be pretty slow, particularly for larger codebases.

Parsing all the LessCSS files takes some time, and we perform a whole

lot of redundant stat calls to get modification times for files. On

a fairly modern computer, this can take around 4 or 5 seconds for a

sizable codebase. For older computers, this can take dozens or more.
To improve performance, this change implements a couple new caches: one

for the list of imported files, and one for modification times.
Any time we want to get a list of files, we check the import files

cache. If not found, we build the new list and stick it in the cache.
Any time we want an mtime, we check the mtime cache, populating that

from a stat call if needed. The way we do this is faster than the

default way that Pipeline's compiler does things, as we're performing

one os.path.getmtime() and not an os.path.exists() +

os.path.getmtime().
If any file (a file we're compiling or a dependency) is outdated, we

invalidate all import file caches that reference it, ensuring the list

of imports will be recalculated.
The list of imported files are always checked individually for outdated

timestamps, whether or not we ran the script to get that list. The mtime

cache expires after 10 seconds, which is just enough to cut back on

lookups during compilation without much risk of actually missing

modifications.
This can help a little with an initial page load, since we're cuttingA

back on the stats, but really improves subsequent page loads, since

there's very little to do until something changes.

Unit tests pass on all supported versions of Python.
Manually tested. Inserted timing code in Pipeline to verify how long

each bundle's outdated checks were taking. Verified that the initial

page load took about the same as before (slight reduction on my system)

but that subsequent page loads were very noticeably faster.