Move core to its own folder and enable Monorepo Build

[dsmilkov]

Move core to its own folder tfjs-core and enable Monorepo build system where each package build runs only if the PR touches a file in that package folder.

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This change is 

[nsthorat]

Reviewed 41 of 369 files at r1.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @dsmilkov and @nsthorat)

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package.json, line 6 at r1 (raw file):

    "rimraf": "~2.6.2",
    "shelljs": "~0.8.3",
    "ts-node": "~7.0.0",

I think you dont need this

[dsmilkov]

Reviewable status: complete! 1 of 1 approvals obtained

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package.json, line 6 at r1 (raw file):

Previously, nsthorat (Nikhil Thorat) wrote…

I think you dont need this

Removed.