cmd/go: force -coverpkg main packages to be built as libraries

This fixes TestScript/cover_pkgall_multiple_mains, which started
failing after CL 174657.

When compiling main packages with coverage instrumentation
(e.g., for -coverpkg all), we now pass -p with the full import path
instead of '-p main'. This avoids link errors
'duplicate symbol main.main (types 1 and 1)'.

Fixes #31946

Change-Id: Id147527b1dbdc14bb33ac133c30d50c250b4365c
Reviewed-on: https://go-review.googlesource.com/c/go/+/176558
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/load/test.go

@@ -353,9 +353,10 @@ Search:
 // preal, packages that import the package under test should get ptest instead
 // of preal. This is particularly important if pxtest depends on functionality
 // exposed in test sources in ptest. Second, if there is a main package
-// (other than pmain) anywhere, we need to clear p.Internal.BuildInfo in
-// the test copy to prevent link conflicts. This may happen if both -coverpkg
-// and the command line patterns include multiple main packages.
+// (other than pmain) anywhere, we need to set p.Internal.ForceLibrary and
+// clear p.Internal.BuildInfo in the test copy to prevent link conflicts.
+// This may happen if both -coverpkg and the command line patterns include
+// multiple main packages.
 func recompileForTest(pmain, preal, ptest, pxtest *Package) {
 	// The "test copy" of preal is ptest.
 	// For each package that depends on preal, make a "test copy"
@@ -386,6 +387,7 @@ func recompileForTest(pmain, preal, ptest, pxtest *Package) {
 			p = p1
 			p.Target = ""
 			p.Internal.BuildInfo = ""
+			p.Internal.ForceLibrary = true
 		}
 
 		// Update p.Internal.Imports to use test copies.