mod/modpkgload: Make LoadPackages support nil shouldIncludePkgFile

The docs already state:

// The shouldIncludePkgFile function is used to determine whether a
// given file in a package should be considered to be part of the build.
// If it returns true for a package, the file's imports will be followed.
// A nil value corresponds to a function that always returns true.

However the implementation didn't match this, and instead crashed if
`shouldIncludePkgFile` is nil. Now fixed.

Change-Id: I5610a38335625b0a74363a00cf8b001f550f608e
Signed-off-by: Matthew Sackman <[email protected]>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1217675
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: cuematthew

internal/mod/modpkgload/pkgload.go

@@ -182,6 +182,9 @@ func LoadPackages(
 	rootPkgPaths []string,
 	shouldIncludePkgFile func(pkgPath string, mod module.Version, fsys fs.FS, mf modimports.ModuleFile) bool,
 ) *Packages {
+	if shouldIncludePkgFile == nil {
+		shouldIncludePkgFile = func(pkgPath string, mod module.Version, fsys fs.FS, mf modimports.ModuleFile) bool { return true }
+	}
 	pkgs := &Packages{
 		mainModuleVersion:    module.MustNewVersion(mainModulePath, ""),
 		mainModuleLoc:        mainModuleLoc,