Dep: Adapt goimports' fast filepath.Walk impl for ListPackages

[fabulous-gopher]

_From @sdboyer on March 7, 2017 14:44_

goimports has an optimized implementation of filepath.Walk() that we should adapt for ListPackages().

_Copied from original issue: sdboyer/gps#180_

[fabulous-gopher]

_From @jstemmer on April 1, 2017 22:19_

I've had a quick look at the fastWalk implementation and it looks like it should be pretty easy to replace the current filepath.Walk implementation with this faster one. Some quick testing shows a consistent improvement in ListPackages performance, I can clean it up and prepare a PR.

Can we just copy the source into this repo with the licensing headers intact or is there something else (license-wise) that I should keep in mind?

[fabulous-gopher]

_From @sdboyer on April 1, 2017 23:38_

Ugh, licenses. We're already trying to figure that out in the reverse direction - golang/dep#300. Maybe it's best to just wait until after we finish that, as the licenses should then be compatible.

Also, a note - we may need it to be somewhat different from goimports' version, as our symlink needs may be a bit different - see #157 and #177.

[fabulous-gopher]

_From @jstemmer on April 1, 2017 23:48_

Sure, I'll hold off on sending it for now and will keep it around in a local branch until gps is merged into dep.

I was vaguely aware that symlinks were being discussed in a couple of places but tried to avoid those discussions so far. I'll have a look through those issues and see what needs to be changed, thanks for linking them.

[fabulous-gopher]

_From @sdboyer on April 2, 2017 1:21_

Sure, I'll hold off on sending it for now and will keep it around in a local branch until gps is merged into dep.

SGTM

I was vaguely aware that symlinks were being discussed in a couple of places but tried to avoid those discussions so far.

That seems like a good act of self-care. I am _truly_ coming to despise symlinks.

[sdboyer]

@jstemmer ok, this one should be ready for your attention now 😄

[jstemmer]

Great! I'll move my branch over soon and see what else was left to be done wrt symlinks before sending a PR.

[sdboyer]

@jstemmer i think a direct merge may not be possible because there was a filter-branch op that changed commit history...but either way, shouldn't be too bad. thanks!

[sdboyer]

@jstemmer pinging, just to see if you maybe have time to look at this again 😄

[jstemmer]

I've been busy lately, but I'll try to find some time to pick this up soon.

[sdboyer]

/cc @karrick, re: godirwalk

[karrick]

I have a tested branch of dep that uses https://github.com/karrick/godirwalk in ListPackages. After #1084 is merged, I'll submit a PR for this issue as well.