LICENSE selection is mildly unclear

Hello awesome people!

I got roped in to looking at this library due to it being a transitive dependency of another dependency I'm using, and I wanted to describe a bit of the experience around figuring out exactly what license applies to this project:

https://github.com/google/re2j/blob/master/build.gradle#L116-L119

The `Go License` best I can tell is not a known SPDX identifier, which makes things a bit unclear when first glancing at stuff. Once you dig a bit, it becomes clear it's a BSD3 style license (only one word really changed from the BSD3 license, HOLDER -> OWNER (screencap attached).

At any rate, the LICENSE file itself is on this repo, and then you link to an external LICENSE, the Go License (which is more or less BSD3). It is a little difficult right from the get go figuring out which applies, etc... yada yada yada, I'm not a lawyer, no interest in becoming one :)

It would seem like at least using an SPDX identifier would be a good step, or seeing if there are truly any differences to the BSD3 license (English is tough and HOLDER -> OWNER is probably a big change if you ask the right person).