Improve performance by avoid fetching unnecessary files

[khanguy00]

Resolves #32

Approach

When walking into a CSS, the plugin will do 2 things separately:

1. Collect definitions @value.
2. Resolve all definitions when needed.

Weakness

• If we have 2 import statements that refer to the same CSS file, the plugin will walk into that file twice.

[khanguy00]

The newest commit 7cbe827 now integrates the caching idea from #34 to make this process potentially faster.

The idea of caching is:

1. We cache the result in walkFile to avoid walking the same file again (same as Cache file definitions (#34), closes #32 #34, thanks to @WearyMonkey)
2. In the cached result, there may be the resolved value or the raw definition
3. The next time we hit the same file, we retrieve the required definitions from the cache and re-evaluate if needed.

[princed]

Thank you @khanguy00 and @WearyMonkey, I'll try and have a look later today.
Does it mean we can close #34?

[khanguy00]

Does it mean we can close #34?

I think so. Unless you're about to decline this PR and accept that instead. 😂

[princed]

I've decided to go with Toby's solution as it's much simpler and still solves the problem. Please feel free to reopen if/when caching isn't enough. Thank you!

[WearyMonkey]

@khanguy00 it might be interesting to compare your change against the baseline of my change. If there is an improvement, then it might be worth submitting this as a follow on fix.