Support || (or) in build-depends

Currently build-depends don't support "or" between package constraints. All the package constraints are implicitly and-ed together. This means that users have to encode or-constraints using flags, which is verbose and can be more error prone (e.g. users might set the flag to manual).

Consider the planned split of network into network and network-uri, where the `Network.URI` module is moved out of network into network-uri. Users that want to use that module need to specify the following dependency:

```
flag network-uri
description: Get Network.URI from the network-uri package.
default: True

library
if flag(network-uri)
build-depends:
network >= 2.6 && network-uri >= 2.6
else
build-depends:
network < 2.6
```

What if we could instead write

```
library
if flag(network-uri)
build-depends:
network >= 2.6 && network-uri >= 2.6 || network < 2.6
```

(The syntax probably need to be different, as build-depends already supports `||`, with a different meaning.)

The implementation could still be in terms of flags.