Filesystem: Provide the string writeFile overload at the interface level ...

The string overload is just a convenience function and there is little
chance that it wll ever need to be overloaded.

MockFS: Simplify existsFile / existsDirectory

MockFS: Simplify implementation of setAttributes / setTimes ...

Hoist the lookup from FSEntry to MockFS to reduce the duplication.

Refactor filesystem abstraction to and use it in production ...

The filesystem abstraction needs to be known to production to avoid
too much code duplication. So far we have used functions in
`PackageManager` that the `TestPackageManager overrides.

With this change, we can remove them, and instead just pass an interface.
On the long run, we should be able to do dependency injection simply
by passing a different object (or overriding) the `Dub` class.

An initial attempt made `FSEntry` inherit from the `Filesystem` interface,
however that attempt was doomed as it conflates two different abstractions:
`FSEntry` is a "node" in the filesystem, which we want to act on in one
of multiple ways: move to another node, or take an action. On the other hand,
`Filesystem` is a more generic interface and takes a path as most of its
function's argument. A `path` and an `FSEntry` have a lot of overlap
(they are a reference to a node in a tree) and the resulting interface
would have been very clunky.

Trivial: Remove internal deprecations ...

Neither commandline not dub.internal are part of the public API for the library,
hence those deprecations were unnecessary.

Move the I/O part of Project.loadSelections to PackageManager ...

By doing so we can remove a sizeable amount of duplication as well
as the last `Dub.loadPackage` override. The resulting API is not
ideal, for example it doesn't allow to distinguish between parsing
errors and non-existent file, but will have to do for now.

Merge remote-tracking branch 'upstream/stable' into merge_stable

Fix regression in module-declarations parser

Add filterInPlace utility function. ...

Note: this function should probably be added to Phobos eventually.

Fix bug

Handle non-ASCII characters

Handle /++ ++/

Handle underscores

Speed up module parsing with hand-written parser

Trivial: Simplify ensureDirectory ...

mkdirRecurse will not do anything if the directory already exists,
so there is no need to call existsDirectory.

Fix ubyte[] download overload by avoiding a cast ...

We were using the default template parameter, `char`,
and as the documentation for `get` states:
> If asking for char, content will be converted from
> the connection character set (specified in HTTP
> response headers or FTP connection properties,
> both ISO-8859-1 by default) to UTF-8.

However we are downloading a binary so converting
it to UTF-8 will fail. Especially if we intend
to cast it to `ubyte[]` afterwards.

Trivial: Print the Exception message in retryDownload ...

This makes the source of the issue more obvious when using
the verbose flag.

Remove FileInfo.timeCreated ...

As it is not available on all platforms, we can't really rely
on it for code, and it is indeed not used anywhere.

Introduce readText and make stripUTF8BOM private

Tests: Fix writeFile implementation with vibe-d ...

This was triggering an assert error with vibe-d path.
The recursion logic was a bit murky, so the function
is now split in 3 different part and better documented.

Make all overloads of download private ...

Since this is not used directly anywhere inside of dub,
make it private so it can be later refactored.

Update configy to the latest HEAD

CI: Move single checks to Github, remove dead code ...

Remove support for very old frontend - we were not testing it anyways.
Move the useful checks to another job so they run once.

Improve unittest framework to have a virtual FS ...

This gets the unittest framework closer to the actual behavior,
and allows us to mock basic FS operations such as mkdir and writeFile.
Further improvements can be made to support more operations,
and to have package setup done before `TestDub` instantiation.

Configy: Synchronize with upstream ...

Bring in some documentation improvements and PR48 bugfix.

Build with -preview=in ...

This reduces the number of deprecations issued in 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

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.

Don't wastefully execute everything via a shell ...

We should only ever use the shell if we want to use the shell's
features, such as pipes, non-trivial redirection, or shell language
features (branches/loops/etc.).  Otherwise, executing a program via
the shell is pointless and wasteful.