build: make packaging more flexible

[devversion]

• Makes the packaging more flexible by no longer including checks that are specific to the Angular Material project (e.g material => lib mapping)
• Updates the code to be more readable by making the packaging more object orientated (TS compilation helpers shouldn't be part of the gulp task generation)
• Avoids repeating the path joining ~100x times by just passing the build package instance around.

[jelbourn]

What do you you think of making the second constructor arg a config / options object so that it looks like

export const materialPackage = new BuildPackage('material', {
  deps: [cdkPackage],
  root: join(buildConfig.packagesDir, 'lib'),
});

?

[devversion]

I wanted to avoid something like that. It's really a special case that we want to overwrite the packageRoot here and it should not really be done through the build package.

Normally the build config is responsible for changing such things, but since this is really a special case I think this would be fine (it's just a single line; which is pretty clear IMO)

[jelbourn]

I don't feel strongly either way

[jelbourn]

LGTM

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