cue/load: respect build tags when determining package

We should respect build tags when deciding what possible
packages to choose where none is specified in the command
line.

Fixes #3795
Signed-off-by: Roger Peppe <[email protected]>
Change-Id: I5e604eb0ac83111c5fb824c319b88eb0eede6845
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1211243
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>

Author: rogpeppe

cmd/cue/cmd/testdata/script/pkg_resolution_multiple_packages_build_attr.txtar

@@ -11,15 +11,15 @@ stderr '^found packages "x" \(x.cue\) and "y" \(y.cue\) in ".*x"$'
 # build attributes.
 
 cd x
-! exec cue eval
-cmp stderr $WORK/eval-x.golden
+exec cue eval
+cmp stdout $WORK/eval-x.golden
 
 -- eval-all.golden --
 x: 5
 // ---
 x: 5
 -- eval-x.golden --
-found packages "x" (x.cue) and "y" (y.cue) in "x"
+x: 5
 -- cue.mod/module.cue --
 module: "mod.com"
 language: version: "v0.9.0"

cue/load/search.go

@@ -436,6 +436,10 @@ func appendExpandedUnqualifiedPackagePath(pkgPaths []resolvedPackageArg, origp s
 		if err != nil {
 			return nil, err
 		}
+		if err := shouldBuildFile(f.Syntax, tg.tagIsSet); err != nil {
+			// Later build logic should pick up and report the same error.
+			continue
+		}
 		pkgName := f.Syntax.PackageName()
 		// 2. if pkgQual is "_", it looks for a package file with no package name.
 		// 3. if there's a package named after ip.Qualifier it chooses that