This commit removes the 'ExTest' constructor from 'ExampleDependency'.
Test dependencies are now represented using the same
'ExampleDependency' constructors as library dependencies.  The only
difference between dependencies of different components is that they
are placed under different keys in 'D.C.ComponentDeps.ComponentDeps'.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

A package path now consists of a namespace and a qualifier. The namespace is
either DefaultNamespace or Independent _i_, for some _i_; this is used for
independent top-level goals. Then the qualifier is either Unqualified
(default), Setup _pn_ for the setup dependencies of package _pn_, or Base _pn_,
for a dependency on base by package _pn_ (used only when we detect the presence
of a base shim). Qualifiers are not tested anymore.

This avoids non-termination in the solver. The unit tests now pass.

In #3170 we introduced a cycle check to the solver. This check is necessary to
reject cycling solutions (which would previously have resulted in an internal
error when we verify the install plan). However, this by itself is not
sufficient. If we have a cycle through setup dependencies, the solver loops
because it starts building an infinite tree. This is explained in detail in

In this commit we just add some unit tests that provide a minimal example that
exposes the bug.

When solving, we now discard plans that would involve packages with a
pkgconfig-depends constraint which is not satisfiable with the current
set of installed packages (as listed by pkg-config --list-all).

This fixes https://github.com/haskell/cabal/issues/3016.

It is possible (in principle, although it should be basically impossible
in practice) that "pkg-config --modversion pkg1 pkg2... pkgN" fails to
execute for various reasons, in particular because N is too large, so
the command line becomes too long for the operating system limits.

If this happens, revert to the previous behavior of accepting any
install plan, regardless of any pkgconfig-depends constraints.

This default is consistent with Cabal.

'DSL.exResolve' now takes a 'Maybe [Extension]' for supported extensions and
a 'Maybe [Language]' for supported languages.  'Nothing' means that
extensions/languages are not checked by the solver, and 'Just []' means that
no extensions/languages are allowed.

This also includes modifications to the solver testing DSL and the
testing functions.

This is necessary for merging PR #2732.

Addresses https://github.com/haskell/cabal/pull/2500#commitcomment-10817840.

This address @23Skidoo's comment
https://github.com/haskell/cabal/pull/2500#issuecomment-8703532

Since we didn't really have a unit test setup for the solver yet, this
introduces some basic tests for solver, as well as tests for independent goals
specifically.