🏗 Always build amp-loader and amp-auto-lightbox (adapted from #24137)

[rsimha]

This PR picks up where #24137 left off.

Highlights:

• Always include amp-loader and amp-auto-lightbox in the list of extensions to be built
• Refactor getExtensionsToBuild() and buildExtensions()
• Remove the defunct options.CompileAll
• Use options.compileOnlyCss to determine if buildExtensions() should build just CSS (for all extensions)
• Minor clean up

Addresses #24137 (review)
Closes #24137

[rsimha]

@sparhami Here are some things to try locally with this PR:

gulp --noextensions
gulp --extensions=amp-foo,amp-bar
gulp
gulp dist --noextensions
gulp dist --extensions=amp-foo,amp-bar

amp-loader and amp-auto-lightbox should be built in all these cases.

[sparhami]

Just a note, this PR removes some functionality I have relied on in the past (using both extensions_from and extensions together), which I have found useful when working on multiple extensions at once.

[rsimha]

Just a note, this PR removes some functionality I have relied on in the past (using both extensions_from and extensions together), which I have found useful when working on multiple extensions at once.

Gotcha. Should be easy to reorganize the if clauses to restore this.

Edit: Fix available in #24227