PackageManager.Location: Split packages registered locally and read from path ...

A Location, having a default search path (the package path),
and some extra search path, optionally, previously stored
all of them in the same array. However, this creates problems,
as we want to move to lazy load the packages in the default
search path, while we can't lazy load packages in non-default ones.

Trivial: Fix some style issues

PackageManager: Simplify logic around refreshing packages ...

Passing an empty array will lead to the same behavior
as using a boolean, so we can remove the parameter.

Trivial: Rename refresh_existing_packages parameter to refresh

Trivial: Reduce noise and allocation while reading local-packages.json

Trivial: Improve dub.packagemanager : Location documentation

Deprecate overrides: local-overrides.json, add-override, remove-override ...

As explained in the changelog, overrides were a solution to a system
with version-only dependencies, which is long gone.

PackageManager: Move scanPackageFolder to Location ...

A simple move for now to keep it simple,
the breach of abstraction will be resolved in a latter commit.

PackageManager.scanPackageFolder: Do some cosmetic fixes

PackageManager: Move class' extra searchPath/Package to an internal location ...

Those fields have been aggregated in Location and them being in `PackageManager`
stands out as a double duty (the `PackageManager` is both a `Location`
and act as a `Location` manager). By using a `Location` for them we can
start to streamline the code.

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`;

Trivial: Convert some spaces to tabs

PackageManager: Deprecate overloads of getPackage with string version

Call file.isDir once in hashPackage

Use SHA256 and better namings in hashPackage

PackageManager.getBestPackage: Introduce Version[Range] overloads

PackageManager: Move routine to write local pkg list to Location struct

PackageManager: Move routine to write override in Location struct

Trivial: Remove redundant cast

PackageManager: Move Location-specific code to Location type ...

Because having a nested function that looks up an index in a field
and goes through this indirection all the time doesn't seem like
the best design anymore.

Trivial: Document PackageManager.Location, add attributes

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.

Trivial: Pass a Repository to loadGitPackage

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.

Trivial: Update comments on PackageManager ...

The previous comments were a bit misleading.

Rename private packagemanager : Repository to Location ...

Avoid ambiguity with dependency : Repository and matches PlacementLocation.

Deprecate duplicated enum LocalPackageType -> PlacementLocation ...

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

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)