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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[googlebot]

CLAs look good, thanks!

[artonge]

Working example in this repo on the working_example branch.

[artonge]

npm run test-large failed for unknown reason. Can a maintainer help sorting this out ?

[filipesilva]

Please see my response on #861 (comment) as to why we do not want to add this right now.