This is a followup to CL 199840 and CL 203820. Cumulatively, they caused
a previously known bug to trigger more often while also nearly fixing it.

This change is a small fixup to CL 199840 that resolves the known bug
and prevents it from causing an additional regression in Go 1.14.

Part 1

The intention in CL 199840 was to return the same error that 'go list'
reported when the package wasn't located, so an early return was added.
However, to determine whether the package was located or not, p.Dir was
unintentionally checked instead of dir.

p is initialized to &Package{ImportPath: path} at top of Context.Import,
and its Dir field is never set before that line in importGo is reached.
So return errors.New(errStr) was always executed whenever errStr != "".

Originally, in CL 125296, the "go list" invocation did not include an
'-e' flag, so it would return a non-zero exit code on packages where
build constraints exclude all Go files, and importGo would return an
error like "go/build: importGo import/path: unexpected output: ...".

CL 199840 added an '-e' flag to the "go list" invocation, but checking
the wrong dir variable caused partial package information to never get
populated, and thus issue #31603 continued to occur, although with a
different error message (which ironically included the location of the
package that was supposedly "not found").

Now that the right dir is checked, issue #31603 is fixed.

Part 2

importGo checks whether it can use the go command to find the directory
of a package. In Go 1.13.x and earlier, one of the conditions to use the
go command was that the source directory must be provided.

CL 203820 made a change such that knowing the source directory was
no longer required:

	 // To invoke the go command,
	-// we must know the source directory,
	 // ...

That meant build.Import invocations where srcDir is the empty string:

	build.Import(path, "", build.FindOnly)

Started using the go command to find the directory of the package, and
started to run into issue #31603 as well. That's the #37153 regression.

Since this change fixes issue #31603, it also fixes issue #37153.

Part 3

There is one more thing. Delete the debugImportGo constant, it's unused.

Updates #26504 (CL 125296)
Updates #34752 (CL 199840)
Updates #34860 (CL 203820)
Fixes #31603
Fixes #37153

Change-Id: Iaa7dcc45ba0f708a978950c75fa4c836b87006f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/218817
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>