make partial mocks an optional thing rather than default thing

[iambumblehead]

the situation reported in this ticket should not happen unless it is specified somehow #51

so instead of something like this

const originalFile = await import( pathtooriginalfile );
const mockedFile = Object.assign({}, originalFile, mockDefinitions);

return mockedFile

we want something like this to happen instead

const originalFile = isPartialMockEnabled && await import( pathtooriginalfile );
const mockedFile = Object.assign({}, originalFile, mockDefinitions);

return mockedFile

[iambumblehead]

maybe the behaviour could be specified by calling esmock this way

const mainWithPartialMockedModules = await esmock.px('../src/main.js', {}, {
  fs : {
    readFileSync : () => 'this function will be merged with other fs definitions';
  }
});

const mainWithStrictMockedModules = await esmock('../src/main.js', {}, {
  fs : {
    readFileSync : () => 'only this function will be defined on fs';
  }
});