Sort extras constraints of dependencies in lockfile

The extras constraints ordering is not guaranteed
on metadata and it causes changes on poetry.lock
file during update process even when the dependency
did not changed.

This patch orders this constraint for lock file
generation changing this behaviour.

poetry/packages/locker.py

@@ -247,7 +247,7 @@ class Locker(object):
             constraint = {"version": str(dependency.pretty_constraint)}
             constraint = {"version": str(dependency.pretty_constraint)}
 
 
             if dependency.extras:
             if dependency.extras:
-                constraint["extras"] = dependency.extras
+                constraint["extras"] = sorted(dependency.extras)
 
 
             if dependency.is_optional():
             if dependency.is_optional():
                 constraint["optional"] = True
                 constraint["optional"] = True

tests/packages/test_locker.py

@@ -246,3 +246,40 @@ A = []
 """
 """
 
 
     assert expected == content
     assert expected == content
+
+
+def test_extras_dependencies_are_ordered(locker, root):
+    package_a = get_package("A", "1.0.0")
+    package_a.add_dependency(
+        "B", {"version": "^1.0.0", "optional": True, "extras": ["c", "a", "b"]}
+    )
+    package_a.requires[-1].activate()
+
+    locker.set_lock_data(root, [package_a])
+
+    expected = """[[package]]
+category = "main"
+description = ""
+name = "A"
+optional = false
+python-versions = "*"
+version = "1.0.0"
+
+[package.dependencies]
+[package.dependencies.B]
+extras = ["a", "b", "c"]
+optional = true
+version = "^1.0.0"
+
+[metadata]
+content-hash = "115cf985d932e9bf5f540555bbdd75decbb62cac81e399375fc19f6277f8c1d8"
+python-versions = "*"
+
+[metadata.files]
+A = []
+"""
+
+    with locker.lock.open(encoding="utf-8") as f:
+        content = f.read()
+
+    assert expected == content