This allows a developer depending on `password` to toggle on and off
support for different password-hashing algorithms at the package level.

The motivator for this is that `cryptonite`'s Argon2 implementation is
broken on M1 Macs (or any `aarch64-darwin` machine), and in the Nix
package set, `cryptonite` has been patched so that any calls to the
Argon2 implementation will cause a compiler error telling the developer
that it is broken. However, as a user of `password`, I don't use Argon2,
and I still want to be able to compile `password` by simply ignoring
Argon2 as a possible hashing algorithm. That's what these flags allow me
to do.

The tests previously relied on the Bcrypt module being exposed to access
certain re-exports from the Internal module. Now that Bcrypt is no
longer guaranteed to be included in the library (i.e. if the brcrypt
cabal flag is disabled), we can't rely on it to be able to run the
tests.

Instead, we add the `src/` folder to the test suite, so that the tests
can directly access the Internal module without needing to expose it to
public users of the library.

This change generates some spurious warnings about "undeclared modules"
when building the tests, but these can safely be ignored. Those modules
are accessed through the `password` library, not via the source code.