stop using listedPackage.IgnoredGoFiles

We used it to detect GOOS-specific packages and ignore their load errors
without having to do a substring search.
However, it turns out that repeatedly loading the string slice
from gob files in the cache is rather slow, particularly since many
Go packages have dozens of GOOS-specific files which can be ignored.

            │      old      │                 new                  │
            │ cached-sec/op │ cached-sec/op  vs base               │
    Build-8     340.3m ± 1%     335.8m ± 2%  -1.32% (p=0.002 n=10)

            │     old     │                new                 │
            │ mallocs/op  │ mallocs/op   vs base               │
    Build-8   35.73M ± 0%   35.09M ± 0%  -1.79% (p=0.000 n=10)

shared.go

@@ -151,7 +151,6 @@ type listedPackage struct {
 
 	Dir             string
 	CompiledGoFiles []string
-	IgnoredGoFiles  []string
 	Imports         []string
 
 	Error *packageError // to report package loading errors to the user
@@ -300,7 +299,7 @@ func appendListedPackages(packages []string, mainBuild bool) error {
 		}
 
 		if perr := pkg.Error; perr != nil {
-			if !mainBuild && len(pkg.CompiledGoFiles) == 0 && len(pkg.IgnoredGoFiles) > 0 {
+			if !mainBuild && strings.Contains(perr.Err, "build constraints exclude all Go files") {
 				// Some packages in runtimeLinknamed need a build tag to be importable,
 				// like crypto/internal/boring/fipstls with boringcrypto,
 				// so any pkg.Error should be ignored when the build tag isn't set.