feat(@ngtools/webpack): add support for multiple entry modules

[artonge]

When the webpack config is building two different apps from two
entry points, one of the apps will not work because metadatas are not
generated. The commit add a new property (entryModules) in
AngularCompilerPluginOptions that permit to regsiter multiple entry
modules.

fix: #861

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[artonge]

I didn't add tests as I didn't knew where to put them and how to test correctly. If you give me some hints I can write some.

[artonge]

Wake up googlebot