Allow multiple trees as input to Filter

[SparshithNR]

• Using fs-merger
  1. Allow filter developers to reduce mergeTree or funneling entire array of trees to one tree and then pass it to filter
  2. Allows input array of trees as input.
• input/output :
  1. Provides good ergonomics and avoids appending the this.outputPath
• Test:
  1. Added additional tests to make sure that Filter can take more than one tree

Example:

class Awk extends Filter {
  constructor(inputNode, search, replace, options) {
    options = options || {};
    super(inputNode, {
      annotation: options.annotation
    });
    this.search = search;
    this.replace = replace;
    this.extensions = ['txt'];
    this.targetExtension = 'txt';
  }
  processString(content, relativePath) {
    return content.replace(this.search, this.replace);
  };
}
module.exports = new Awk(['fixture','fixture_2'], 'test', 'real');

Since we are now allowing users to pass two paths/nodes. We avoid a uncessary merge/funnel.
Before to achive the above result we had to below sinppet.

let fullTree = mergeTree(['fixture','fixture_2'];
module.exports = new Awk(fullTree, 'test', 'real');

[chriseppstein]

I didn't review everything, but I have enough questions to pause at this point.

[stefanpenner]

as implemented fsMerger is still part of the public API for subclasses, this seems sorta strange given the above weakmap usage.

[SparshithNR]

Input output facade is addressed in #187.
We will address allowing multiple trees as input in this PR.

[stefanpenner]

Using this.inputPaths[0] here is breaking encapsulation, post this.input and this.output migration it should be possible to use only the new APIs, what features are missing or different approaches are required to make this statement true.

[stefanpenner]

@SparshithNR ^^

[stefanpenner]

^^

[stefanpenner]

Currently I am worried, that this plugin does not use this.input, did we make a mistake with it? By not using this.input we are making it hard for future optimizations, as ensuring all plugins go through this.input this.output opens many doors when it comes to future optimizations.

[SparshithNR]

We may have to make changes to the fs-merger library to somehow match inputPaths and input to broccoli-persistent-filter where we expect input can be broccoli-node string or an object with prefix/getDestinationPath function.

[SparshithNR]

Closing this as #188 is addressing this