Installing dependencies takes too long (affects both Homebrew and Travis)

[Dids]

Especially on a clean install (brew remove clobber --force && brew install --force --HEAD clobber), it takes 2 minutes to install clobber, even on the most powerful of systems.

Either this means that the current formula is ignoring the vendor/ folder, or the packr2 binary installation is what's actually taking so long.

In any case, this needs to be resolved, as everything should be bundled with the project, then built and installed, which should definitely take less than a minute total (on average anyway).

• Fix dependency caching for Travis
• Fix dependency caching for Bitrise
• Fix dependency caching for Homebrew

[Dids]

Update: I'm fairly certain that even Travis builds are still downloading dependencies, and perhaps partially ignoring the vendor/ folder?

[Dids]

More information: make deps takes ~15 seconds in a Travis build, so it's definitely not relying entirely on the vendored modules, as it should. Caching the entire GOPATH didn't do much to improve this either.

[Dids]

Specifically on Go v1.9, make deps takes only a second or two, but anything higher (including v1.10 and v1.11) takes over 25 seconds (70 seconds without caching).

I'm guessing this comes from v1.9 using the vendor/ folder, which anything above ignores it and downloads the packages defined in the Go module file.

[Dids]

Attempting to solve for Travis by caching $GOPATH/{bin,pkg,src}, but ignoring anything clobber related.

UPDATE: That seems to have done it, Travis builds are now a lot faster due to the caching (and due to the Makefile now checking if packr2 is installed or not)