This CL makes "go install" behave the way many users expect:
install only the things named on the command line.
Future builds still run as fast, thanks to the new build cache (CL 75473).
To install dependencies as well (the old behavior), use "go install -i".

Actual definitions aside, what most users know and expect of "go install"
is that (1) it installs what you asked, and (2) it's fast, unlike "go build".
It was fast because it installed dependencies, but installing dependencies
confused users repeatedly (see for example #5065, #6424, #10998, #12329,
"go build" and "go test" so that they could be "fast" too, but that only
created new opportunities for confusion. We also had to add -installsuffix
and then -pkgdir, to allow "fast" even when dependencies could not be
installed in the usual place.

The recent introduction of precise content-based staleness logic means that
the go command detects the need for rebuilding packages more often than it
used to, with the consequence that "go install" rebuilds and reinstalls
dependencies more than it used to. This will create more new opportunities
for confusion and will certainly lead to more issues filed like the ones
listed above.

CL 75743 introduced a build cache, separate from the install locations.
That cache makes all operations equally incremental and fast, whether or
not the operation is "install" or "build", and whether or not "-i" is used.

Installing dependencies is no longer necessary for speed, it has confused
users in the past, and the more accurate rebuilds mean that it will confuse
users even more often in the future. This CL aims to end all that confusion
by not installing dependencies by default.

By analogy with "go build -i" and "go test -i", which still install
dependencies, this CL introduces "go install -i", which installs
dependencies in addition to the things named on the command line.

Fixes #5065.
Fixes #6424.
Fixes #10998.
Fixes #12329.
Fixes #18981.
Fixes #22469.

Another step toward #4719.

Change-Id: I3d7bc145c3a680e2f26416e182fa0dcf1e2a15e5
Reviewed-on: https://go-review.googlesource.com/75850
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>