fbcode_builder: getdeps: add boolean expression parser

Summary:
As part of folding getdeps into fbcode_builder, this
expression parser is needed to allow constrained and deterministic
conditionals in the manifest file format.

For example, the watchman manifest will use this cargo-inspired syntax
for system dependent sections:

```
[dependencies]
folly

[dependencies.not(os=windows)]
thrift
```

Reviewed By: sinancepel

Differential Revision: D14691014

fbshipit-source-id: 080bcdb20579da40d225799f5f22debe65708b03

build/fbcode_builder/getdeps/test/expr_test.py

@@ -0,0 +1,42 @@
+#!/usr/bin/env python
+# Copyright (c) 2019-present, Facebook, Inc.
+# All rights reserved.
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree. An additional grant
+# of patent rights can be found in the PATENTS file in the same directory.
+
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+import unittest
+
+from ..expr import parse_expr
+
+
+class ExprTest(unittest.TestCase):
+    def test_equal(self):
+        e = parse_expr("foo=bar")
+        self.assertTrue(e.eval({"foo": "bar"}))
+        self.assertFalse(e.eval({"foo": "not-bar"}))
+        self.assertFalse(e.eval({"not-foo": "bar"}))
+
+    def test_not_equal(self):
+        e = parse_expr("not(foo=bar)")
+        self.assertFalse(e.eval({"foo": "bar"}))
+        self.assertTrue(e.eval({"foo": "not-bar"}))
+
+    def test_bad_not(self):
+        with self.assertRaises(Exception):
+            parse_expr("foo=not(bar)")
+
+    def test_all(self):
+        e = parse_expr("all(foo = bar, baz = qux)")
+        self.assertTrue(e.eval({"foo": "bar", "baz": "qux"}))
+        self.assertFalse(e.eval({"foo": "bar", "baz": "nope"}))
+        self.assertFalse(e.eval({"foo": "nope", "baz": "nope"}))
+
+    def test_any(self):
+        e = parse_expr("any(foo = bar, baz = qux)")
+        self.assertTrue(e.eval({"foo": "bar", "baz": "qux"}))
+        self.assertTrue(e.eval({"foo": "bar", "baz": "nope"}))
+        self.assertFalse(e.eval({"foo": "nope", "baz": "nope"}))