chore(sourcemap): convert inputSourceMap to plain js object

[thebanjomatic]

@babel/core recently made a change where the sourcemap is no longer a plain js
object, but instead is a class SourceMap. This breaks when the coverage data
gets passed into valueToNode by istanbul-lib-instrument because the coverage
data is no longer entirely plain json objects (this.cov.toJSON() just returns
the data and doesn't do any conversion).

In this PR we are checking if the input sourcemap is a plain js object, and
converting it if it is not.

Note that this does seem like the more appropriate place to make the fix since it
is istanbul that has the requirement that the coverage data is a plain javascript object
and the patch applied here babel/babel#14283 is more of a
temporary workaround.

istanbuljs/istanbuljs#672

[bcoe]

@thebanjomatic I think this problem is better fixed here, if I'm following:

istanbuljs/istanbuljs#662

The only thing blocking #662 from landing is that someone needs to write a test. I would happy accept a PR for a test, if you'd like to contribute (you can branch from #662).