Skip modified compilation for namespaces if solc < 0.8.20

[ericglau]

To support namespaced storage validations, the Hardhat Upgrades plugin overrides the Hardhat compile phase to also make a modified copy and compilation of all contracts so that any namespaced structs' storage layouts can be extracted. This requires use of the AST to detect source locations and modify the source code accordingly.

In old versions of Solidity such as 0.5.x, the AST does not provide source locations for certain nodes, such as documentation nodes. This leads to an error such as #891.

To avoid this, we should skip the modified copy/compilation step for namespaced storage if the solc version is < 0.8.20.

Note that for versions of solc < 0.8.20, namespaced storage validation is not supported anyways because the Solidity compiler does not output struct documentation in the AST. We already throw an error if we detect namespaces being used with solc < 0.8.20, so the modified namespaced compilation is not needed for these cases.

Fixes #891

[ernestognw]

LGTM