This transforms the virtualized directory (build.Context).Dir to a physical one,
for proprietary build systems that distinguish them.
LGTM=gri
R=gri
CC=golang-codereviews
https://golang.org/cl/116230043
Files that import "C" are not valid Go source files and
require preprocessing. Until now, the loader has simply
hard-coded CGO_ENABLED=0 (in effect) which causes go/build to
use build tags select pure Go implementations where they exist
(e.g. in $GOROOT). Where they don't (e.g. arbitrary user
code) this leads to masses of spurious type errors.
(Reported by Guillaume Charmes, private correspondence.)
This change causes the loader to invoke the cgo preprocessor
on such files and to load the preprocessed files instead,
using the original names. This means that the syntax offset
position information is garbage, although thanks to //line
directives, the line numbers at least should be good.
See comment in cgo.go for details.
This CL changes the loader's default behaviour and may make it slower.
CGO_ENABLED=0 enables the old behaviour.
Tested via stdlib_test, which now loads all standard packages
using cgo, and also exercises CGO_ENABLED=0 for "net" and "os/user".
LGTM=gri
R=gri, rsc
CC=golang-codereviews, guillaume.charmes
https://golang.org/cl/86140043
Alan Donovan