core(computed-artifacts): convert to named exports

[connorjclark]

ref #12689

• the lantern metric computed artifact names are pretty long. most imports would abbreviate them. currently the PR uses the long version, but the classes themselves could be abbreviated. ex: LanternLargestContentfulPaint could be renamed LanternLCP
• the exported symbols could be XComputed instead of X... not very sure about this one. The caller kinda cares that the thing is a computed artifact insomuch that it needs to call .request (and not something else by accident, missing out on the cache). The previous importing code was inconsistent on this, so I went with X for brevity. No one wants to see NetworkRecordsComputed everywhere, for example.
• a few computed artifacts are named the same as the audit that uses them, so in those instances they get imported as XComputed.

[adamraine]

In general this change looks good. A couple of consistency things:

• ComputedX vs XCompeted
• ComputedFcp vs ComputedCLS

Would be nice to pick one and stick to it but I don't think we need to do that here.