Trivial: Use public interface, not field names, in unittest ...

This would make moving this unittest to another module easier.

Trivial: Rename computePkgSuppliers to makePackageSuppliers ...

This is consistent with the naming of other factory functions.

Trivial: Shorten a few parameter names ...

The parameter names were repeating the type name,
which in a language like D does not make sense.

Trivial: Make `TestDub` non-final ...

There is no reason to limit inheritance on this class.

Allow to load dub.selections.json before Project instantiation ...

The aim of this commit is to have at least one constructor
for Project that does not perform IO. By moving the IO to the
`Dub` class, we can keep the dependency injection in `Dub`
and `PackageManager` only, and don't have to specialize `Project`.

Trivial: Remove a dependency to std.path ...

Use the Dub versions instead.

Trivial: Merge two related unittest in dub package ...

They both test computePackageSuppliers in the end.

Refactor loadConfig to not rely on class state ...

By making it take a parameter and return a value, we make it
more obvious what is being used and changed, and we document
that rootPath is the only class member being read,
which is a fair assumption for any method of Dub.

Change platform dependency to use a VersionRange ...

Note that this is a breaking change for library users.
It changes the platform dependency to only accept version ranges,
instead of any dependency. The end-users already could only write
VersionRange in there, the type was just too wide.

Trivial: Move TestDub behind a version (unittest) ...

As it should not be compiled in the final binary for releases,
it's just a class to help with unittests.

Dub: Make determineDefaultCompiler configurable ...

By making it an idempotent (albeit not pure) function,
we allow other classes to override it with their own logic.
One such example would be returning an hardcoded path every time.

PackageManager: Make field protected, not private ...

This make implementing a different PackageManager possible.

Deprecate string-as-version PackageManager.getBestPackage overload ...

Because using a typed Version or VersionRange is better than using
a string and we want to encourage version validation to be done
earlier than later.

Move dmd version parser to packagerecipe ...

This module is the only one that uses it, so instead of having
it in a place that looks correct but where it is not used,
move it to packagerecipe and make it private.

Trivial: Reduce import scope in dub.recipe.selection

Trivial: Remove dead imports from dub.dependency ...

This module is not performing any IO besides logging,
and doesn't use package_ or utils.

Move data part of dub.platform to dub.data ...

We want to separate the data part, which is exposed to the PackageRecipe,
from the actual platform check code, which is used when building.

Remove logging from dub.dependency ...

The logging was not really useful: One unittest one,
and a spurious one with no action.
There is a lot of value in having dependency be data-only,
not having side effect or much dependency to the rest of dub.

Trivial: Inline unused listDirectory into iterateDirectory ...

In practice we only every iterate directories in a foreach,
which means iterateDirectory is the only function ever used.

Trivial: Remove unused listDirectory string overload

Trivial: Simplify some deprecated code ...

While this code is deprecated, it is confusing as it is
using findPackageFile only to provide a worse error message
than one would have got by simply calling Package.load.

Remove inconsistent error on missing subpackage path ...

The behavior of the code was slightly puzzling: if the package path did
not exist, we could log an error and continue, but if it existed but
was empty, we would error out. To be consistent, simply always error out.

Change PackageSupplier interface to accept VersionRange ...

This is a leftover of the time when everything in dub accepted Dependency.
While the mess had been untangled, doing so on this interface proved harder,
as there is no good way to deprecate it in a fully backward-compatible manner
without introducing a lot of overhead (a new type). Instead, this uses
a middle ground approach, where the external implementers (which most likely
do not exists) would break (although the fix is trivial), but callers would not.

Deprecate unused packageInfoFilenames ...

This is of little value (it's just a map) and not used within dub.

Trivial: Simplify checkDubRequirements ...

Remove a redundant import and streamline control flow.

Trivial: Remove dead existsFile import ...

It is not used in this function.

Remove unused iterateDirectory(string) ...

It was actually unused, because al the call sites were doing a useless
round-trip from NativePath to string to NativePath.
There is no need for it anymore so we can just remove it to avoid
this kind of mistakes in the future.

Move application code for static ctor in main module ...

This code could easily mess with library users as it has
some side effects, so move it somewhere it will only affect
the binary.

Trivial: Clear up some trailing whitespace

Fix issue #2190 - windows: dub won't run if TEMP is unset or empty