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
