cmd/go: copy files from go/build into new modindex package

These files are all copied as is from the go/build package, to files
with the same name in modindex (with the exception of build_read, which
was copied from go/build/read.go).

This is being done so that the next CL can show exactly the changes that
were made against the go/build versions.

Unfortunately, git doesn't recognize these as copies, which is annoying.

Change-Id: I27b05b23dc5ccefe5252956bf75025bd57b36c66
Reviewed-on: https://go-review.googlesource.com/c/go/+/403777
Reviewed-by: Michael Matloob <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Michael Matloob <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>

Author: matloob