add $DPATH environment variable for file locations (#2281) ...

* error out on unknown environment variables

* add env + config for package + settings locations

fix #229

* adjust to review, extend $DPATH test

* review round 2

PackageManager: Deprecate overloads of getPackage with string version

Dub: Move environment read one level up the stack ...

Avoid a unittest modifying the environment,
and pave the way for having a single read of this variable,
instead of once per function call.

Deprecate Dub.getPackageSuppliers ...

As it really should be an implementation details (only called from the ctors),
and the name is misleading as it reads the environment and the config.

Dub: Remove unused m_overrideSearchPaths member ...

It was likely moved in the PackageManager ages ago.

Trivial: Group 'Dub' class ctor/init together

PackageManager.getBestPackage: Introduce Version[Range] overloads

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

Improve searchPackages naming+docs ...

See https://github.com/dlang/dub/commit/ca988572b58d34a4d9384caeabbdb2fd8d7d0cd5#r80490672

Remove `Dub.updatePackageSearchPath` ...

This function was essentially two functions: One when an override was set,
another one for the common case. However, the function is private,
and the number of call sites is limited, so it's better to inline
the call at the caller site. Once that was done, it became obvious
it only ever has an effect once, when the Dub instance is constructed,
as we don't modify this environment variable.
Thus, we can inline it in the constructor, and remove the dead branches.

Integrate Configy colored output with dub's

dub search: Improve text, rename parameter ...

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

Dub.fetch: Remove a redundant getPackage call ...

Since storeFetchedPackage already returns the Package.

Trivial: Deprecate a compatibility overload of Dub.remove

dub.fetch: Do not catch Exception when a local package can't be loaded ...

Given how simple the function being called is,
there is no reason for us to ignore this error.

Trivial: Remove workaround for DMD v2.070

Trivial: Fix indentation of a log statement

Deprecate duplicated enum LocalPackageType -> PlacementLocation ...

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

settings/selections: Only warn on unknown keys, add changelog ...

This will give users the time to adapt, without erroring right away.

Support escaped forward slashes in YAML parser ...

Integrate fix for issue dlang-community/D-YAML#302 .

Move environment read outside of DependencyResolver ...

Reading environment variables from random places in the code makes
the flow harder to follow, and unittesting harder.
This moves one environment reads to the `dub` module,
which performs many of them already.

Parse settings.json using configy ...

This will provide superior error messages to the user,
and match what we already do for `dub.selections.json`.

It also allows us to remove many fields from the `Dub` class.
Those fields existed so that we would not have to re-compute the data
from the JSON DubConfig was storing. Now that we use a simple struct,
and do the aggregation eagerly, all of that complexity can go.

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.

Output improvements in package management ...

(dub fetch and dub remove)

Improved dub init output ...

Remove duplicate message (package created in ...) and using colored tag

More improvements and replacements ...

Adapted generators output to new style, small change for non-selected
packages