cmd/go: refactor load.LoadPackage into other functions

LoadPackage was used to load a *load.Package for a command line
argument, after pattern expansion. It provided two special cases on
top of LoadImport. First, it ensured that "cmd/" packages in GOROOT
were installed in "$GOROOT/bin" or "$GOROOT/pkg/tool". Second, it
translated absolute paths to packages in GOROOT and GOPATH into
regular import paths.

With this change, LoadImport now ensures "cmd/" packages have the
right Target (without the need for a special case) and
search.ImportPaths translates absolute paths.

LoadPackage no longer handles these special cases and has been renamed
to LoadImportWithFlags, since it's still useful for loading implicit
dependencies.

Updates #29758

Change-Id: I9d54036f90c3ccd9b3a0fe0eaddaa7749593cc91
Reviewed-on: https://go-review.googlesource.com/c/go/+/167748


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>