change links architecture

[GiladShoham]

At the moment when doing bvm link we create something like this
{linkName} -> {bvmFolder}/versions/{versionNumber}
for example:
bit -> ~/.bvm/versions/0.0.466
That means then if you install a new version of bit and remove the old one, the folder {bvmFolder}/versions/{versionNumber} doesn't exist anymore.
We do however have references to this folder - the links from any workspace to the core aspects looks like this:
node_modules/@teambit/{coreAspect} -> {bvmFolder}/versions/node_modules/@teambit/{coreAspect} which of course won't exist anymore, so you have to re-link everything.

Instead, it will be better to have a structure like this:
{linkName} -> {bvmFolder}/links/{linkName} -> {bvmFolder}/versions/{versionNumber}
for example:
node_modules/@teambit/{coreAspect} -> {bvmFolder}/links/{linkName}/@teambit/{coreAspect} -> {bvmFolder}/versions/node_modules/@teambit/{coreAspect}

Which means when installing a new version of bit and re-link we will only re-link this part:
before:
{bvmFolder}/links/{linkName} -> {bvmFolder}/versions/{versionNumber}
after (see the newVersionNumber):
{bvmFolder}/links/{linkName} -> {bvmFolder}/versions/{newVersionNumber}

This means no links will be breaking.