Make various related changes to templates, tests, etc. The methodology for
finding files that needed changes was to search through the whole monorepo for
the strings "solc" and "solidity" and then inspect the hits to see whether
changes were needed/appropriate.
Remove `solc` as a dependency in `packages/embark/package.json` so that it's
only a proper dependency in `packages/plugins/solidity/package.json`. Adjust
how the "bundled" `solc` package's version is determined, i.e. inspect the
`package.json` of `embark-solidity` instead of `embark`.
When `solc`'s version is `>=0.6.0` use the [new callback API][api].
[api]: https://github.com/ethereum/solc-js/blob/master/README.md#example-usage-with-import-callback
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