Use configy library to parse dub.json file ...

This will provide a vastly superior UX to the users:
- They will be warned about typos;
- The type information provided will be more accurate and always include file/line infos
  (e.g. no more 'Invalid SemVer versions');
- We can expose a better recipe struct to library users on the long run,
  by separating the actual data DUB uses from what it reads;
- We can be more descriptive about errors / intent, for example by using `SetInfo`;

Dependency: Deprecate path/repository setters ...

There is no real use case for changing a dependency in place,
instead we better let the user overwrite the dependency itself.

Forward flag `--color={on,off}` to compiler calls (#2382) ...

When the user sets the color mode explicitly,
we forward it to the compiler for consistency,
while making sure it doesn't impact the build cache ID.

Rename CLI flag --colors to --color any D code symbols accordingly

PackageManager.getBestPackage: Introduce Version[Range] overloads

Trivial: Rework some control flow for readability

Dub.fetch: Deprecate Dependency overload, only accept Version[range] ...

Because fetching a path-based or repository dependency doesn't
make much sense.

improve some output coloring ...

See #2347

change TAG_WIDTH const to a variable value ...

use it for making error-only output be more readable

Deprecate Dependency.versionSpec ...

All places which assumed that Dependency contained a VersionRange have been culled,
so we can now start removing those properties.

Overrides: Make it clear that only VersionRange can be overriden ...

The code was always assuming overrides were VersionRange,
as trying to do 'dub add-override' would call 'Dependency'
constructor with a string, which forwards to VersionRange.fromString.
However, when using dub as a library, this assumption was much less obvious,
notably because the fields are public.
With this change, we make sure library users do not misuse overrides.

Use std.sumtype for PackageOverride too ...

Like Dependency, we only were using one of those two fields,
but it was not readily obvious before, and we relied on one
of them being empty to read the other one.

make command exit codes more consistent

dub search: Improve text, rename parameter ...

The 'query' name is inaccurate, as we only ever expect a package name.

Revert "Default `dub init` to SDL" ...

This reverts commit 3d0d867.

Default `dub init` to SDL ...

Átila wants to throw out dub.json, so this is the first step to discouraging dub.json

Deprecate duplicated enum LocalPackageType -> PlacementLocation ...

Two enums, both have the same values, in the same order.

Incorporate review by Jan Jurzitza <gh@webfreak.org>.

Address review by WebFreak001.

Keep logging functionality out of the public API as requested in https://github.com/dlang/dub/pull/2124#issuecomment-813016939.

Fix/Update some tests and make some info messages more similar to original version of Dub

Fix rebase errors

Fix really stupid error in previous commit

Replace --no-colors & --force-colors with --colors={auto|on|off}

Add --force-colors option to force colors enabled

Add --no-colors option to disable colors in terminal ...

Minor: changed function name in logging module

Revert logInfo to print a single line after a fetch

Fixed project upgrade message

Output improvements in package management ...

(dub fetch and dub remove)

Now checks if the output is a console or not ...

This allows to disable colors accordingly, to avoid ASCII escape
sequences in piped output.

!!! tested only on Linux.

Also: minor documentation improvements