Allow loading mods from version-specified directory i.e. mods/rift/1.13/

[k-jiang]

Let Rift to try loading mods form the mods/"version"-Rift folder. If empty, fall back to "mods/" instead.
p.s. I removed the "final" flag from variable modsDir, not sure if there are any consequences.

[k-jiang]

OK, I modify the loading process, now the logic is:

1. Load classpath mods, count and log it
2. Check if there are directory named "mods/rift/1.13/". If so, load jar mods from it, count, and log it
3. Create and load jar mods from "mods/rift/", count and log it
4. Create(even though step 3 already created) and load jar mods from "mods/", count and log it

The reason why I chose lower-case for "rift" is to eliminate any case-sensitive scenario and confusion, just like forge.
And users can create their own "1.13" sub-directory if they want. But "rift" will always be created so people know where to organize their mods, making it fool-proof (at least a little bit).

Any ideas?

[0f-0b]

Suggest changes for readability and performance.