implement old linkconstraintinterface for BC

src/Composer/Package/LinkConstraint/EmptyConstraint.php

@@ -12,13 +12,13 @@
 
 namespace Composer\Package\LinkConstraint;
 
-use Composer\Semver\Constraint\EmptyConstraint as BaseEmptyConstraint;
+use Composer\Semver\Constraint\EmptyConstraint as SemverEmptyConstraint;
 
 @trigger_error('The ' . __NAMESPACE__ . '\EmptyConstraint class is deprecated, use Composer\Semver\Constraint\EmptyConstraint instead.', E_USER_DEPRECATED);
 
 /**
  * @deprecated use Composer\Semver\Constraint\EmptyConstraint instead
  */
-class EmptyConstraint extends BaseEmptyConstraint
+class EmptyConstraint extends SemverEmptyConstraint implements LinkConstraintInterface
 {
 }

src/Composer/Package/LinkConstraint/MultiConstraint.php

@@ -12,13 +12,13 @@
 
 namespace Composer\Package\LinkConstraint;
 
-use Composer\Semver\Constraint\MultiConstraint as BaseMultiConstraint;
+use Composer\Semver\Constraint\MultiConstraint as SemverMultiConstraint;
 
 @trigger_error('The ' . __NAMESPACE__ . '\MultiConstraint class is deprecated, use Composer\Semver\Constraint\MultiConstraint instead.', E_USER_DEPRECATED);
 
 /**
  * @deprecated use Composer\Semver\Constraint\MultiConstraint instead
  */
-class MultiConstraint extends BaseMultiConstraint
+class MultiConstraint extends SemverMultiConstraint implements LinkConstraintInterface
 {
 }

src/Composer/Package/LinkConstraint/SpecificConstraint.php

@@ -19,6 +19,6 @@ use Composer\Semver\Constraint\AbstractConstraint;
 /**
  * @deprecated use Composer\Semver\Constraint\AbstractConstraint instead
  */
-abstract class SpecificConstraint extends AbstractConstraint
+abstract class SpecificConstraint extends AbstractConstraint implements LinkConstraintInterface
 {
 }

src/Composer/Package/LinkConstraint/VersionConstraint.php

@@ -19,6 +19,6 @@ use Composer\Semver\Constraint\Constraint;
 /**
  * @deprecated use Composer\Semver\Constraint\Constraint instead
  */
-class VersionConstraint extends Constraint
+class VersionConstraint extends Constraint implements LinkConstraintInterface
 {
 }