Added experimental feature to improve build cache efficiency

Using version identifiers to configure certain features can often lead to
unnecessary rebuilds of dependencies that don't use those version
identifiers. In order to improve the build cache efficiency, dub gained a new
experimental `--filter-versions` switch.

When `--filter-versions` is passed to any build, test, or generate command, dub
will grep for all the version identifiers packages actually use and only apply
those during building. This allows for example to reuse a cached build for a
library between two applications using different version identifiers when the
library isn't using any of those itself.

The following regular expressions used to grep for version identifiers.
---
enum verRE = ctRegex!`(?:^|\s)version\s*\(\s*([^\s]*?)\s*\)`;
enum debVerRE = ctRegex!`(?:^|\s)debug\s*\(\s*([^\s]*?)\s*\)`;
---

For packages that use version identifiers in mixins or auto-generated sources,
the list of applicable version identifiers can be specified explicitly in the
package file.

dub.json:
---
"-versionFilters": ["Have_vibe_d"]
"-versionFilters-posix": ["UseUnixSockets", "UseMMap"]
"-debugVersionFilters": ["ValidateRequests"]
---
dub.sdl:
---
x:versionFilters "Have_vibe_d"
x:versionFilters "UseUnixSockets" "UseMMap" platform="posix"
x:debugVersionFilters "ValidateRequests"
---

Note that the inferred version identifiers are cached and grepping is generally
very fast, so explicitly specifying version identifiers should only be used if
necessary.

Also note that specifying either of versionFilters or debugVersionFilters will
disable inference for both of them.

The reservered version identifier none can be used for packages that don't use
any version identifiers or debug version identifiers at all.

dub.json:
----
"-versionFilters": ["none"]
----
dub.sdl:
----
x:debugVersionFilters "none"
----