Stop using mage:import in custom beats (elastic#14162)

`mage:import` doesn't take into account vendorized dependencies, this is a
problen for custom beats, because by default they have libbeat as a
vendorized dependency. The solution would be to use go modules, but in
the meantime lets remove the use of `mage:import` from the magefiles
intended to be used by custom beats.

generator/beat/{beat}/magefile.go

@@ -10,20 +10,11 @@ import (
 	"github.com/magefile/mage/sh"
 
 	devtools "github.com/elastic/beats/dev-tools/mage"
-	"github.com/elastic/beats/generator/common/beatgen"
-
-	// mage:import
-	"github.com/elastic/beats/dev-tools/mage/target/pkg"
-	// mage:import
 	"github.com/elastic/beats/dev-tools/mage/target/build"
-	// mage:import
-	_ "github.com/elastic/beats/dev-tools/mage/target/common"
-	// mage:import
-	_ "github.com/elastic/beats/dev-tools/mage/target/test"
-	// mage:import
-	_ "github.com/elastic/beats/dev-tools/mage/target/unittest"
-	// mage:import
-	_ "github.com/elastic/beats/dev-tools/mage/target/integtest"
+	"github.com/elastic/beats/dev-tools/mage/target/common"
+	"github.com/elastic/beats/dev-tools/mage/target/pkg"
+	"github.com/elastic/beats/dev-tools/mage/target/unittest"
+	"github.com/elastic/beats/generator/common/beatgen"
 )
 
 func init() {
@@ -66,3 +57,34 @@ func Fields() error {
 func Config() error {
 	return devtools.Config(devtools.AllConfigTypes, devtools.ConfigFileParams{}, ".")
 }
+
+// Clean cleans all generated files and build artifacts.
+func Clean() error {
+	return devtools.Clean()
+}
+
+// Check formats code, updates generated content, check for common errors, and
+// checks for any modified files.
+func Check() {
+	common.Check()
+}
+
+// Fmt formats source code (.go and .py) and adds license headers.
+func Fmt() {
+	common.Fmt()
+}
+
+// Test runs all available tests
+func Test() {
+	mg.Deps(unittest.GoUnitTest)
+}
+
+// Build builds the Beat binary.
+func Build() error {
+	return build.Build()
+}
+
+// CrossBuild cross-builds the beat for all target platforms.
+func CrossBuild() error {
+	return build.CrossBuild()
+}

generator/metricbeat/{beat}/magefile.go

@@ -9,18 +9,13 @@ import (
 	"github.com/magefile/mage/mg"
 
 	devtools "github.com/elastic/beats/dev-tools/mage"
-	"github.com/elastic/beats/generator/common/beatgen"
-	metricbeat "github.com/elastic/beats/metricbeat/scripts/mage"
-
-	// mage:import
-	_ "github.com/elastic/beats/dev-tools/mage/target/common"
-	// mage:import
 	"github.com/elastic/beats/dev-tools/mage/target/build"
 	"github.com/elastic/beats/dev-tools/mage/target/collectors"
 	"github.com/elastic/beats/dev-tools/mage/target/common"
 	"github.com/elastic/beats/dev-tools/mage/target/pkg"
 	"github.com/elastic/beats/dev-tools/mage/target/unittest"
 	"github.com/elastic/beats/dev-tools/mage/target/update"
+	"github.com/elastic/beats/generator/common/beatgen"
 	metricbeat "github.com/elastic/beats/metricbeat/scripts/mage"
 )