[vcpkg] Improve various versioning error messages

[ras0219]

With suggested fixes and additional help links.

See also changes to vcpkg help versioning in help.cpp.

Example 1:

{
    "name": "testing2",
    "version": "0",
    "builtin-baseline": "23306bdfe67d081915c798695a780366775ed288",
    "dependencies": [
        {
            "name": "glfw3",
            "version>=": "3.3-2"
        }
    ]
}

Before:

$ vcpkg.old install
Version conflict on glfw3@3.3-2: baseline required 3.3.2

After:

$ vcpkg.new install
Error: Version conflict on glfw3:x64-linux: baseline required 3.3.2 but vcpkg could not compare it to 3.3-2

The two versions used incomparable schemes:
    "3.3-2" was of scheme string
    "3.3.2" was of scheme string

This can be resolved by adding an explicit override to the preferred version, for example:

    "overrides": [
        { "name": "glfw3", "version": "3.3-2" }
    ]

See `vcpkg help versioning` for more information.

Example 2:

{
    "name": "testing2",
    "version": "0",
    "builtin-baseline": "23306bdfe67d081915c798695a780366775ed288",
    "dependencies": [
        "glfw"
    ]
}

Before:

$ vcpkg.old install
Cannot resolve unversioned dependency from top-level to glfw without a baseline entry or override.

After:

$ vcpkg install
Error: Cannot resolve a minimum constraint for dependency glfw from testing2:x64-linux.
The dependency was not found in the baseline, indicating that the package did not exist at that time.
This may be fixed by providing an explicit override version via the "overrides" field or by updating the baseline.
See `vcpkg help versioning` for more information.