🏗 Wait on pending builds during gulp --lazy_build

[rsimha]

Today, if a test webpage requests a JS file more than once during gulp --lazy_build, concurrent builds are triggered for the same file, leading to a race.

This PR eliminates the race as follows:

• Changes the function signature of doBuildJs() to match that of doBuildExtension()
• Refactors repeated code in lazyBuildExtensions() and lazyBuildJs() into a helper function lazyBuild() that calls the correct build function if necessary
• Adds a mechanism to lazyBuild() to keep track of and wait for pending builds before serving

This implements the "luxury" version of lazy building mentioned in #24138 (review)

Addresses #24141 (comment)
Partial fix for #24141
Follow up to #24138, #24152, and #24199

[cramforce]

Lovely

[cramforce]

However, one more big update (can be in followup)

Here https://github.com/ampproject/amphtml/blob/master/build-system/tasks/helpers.js#L394

it would be perfect if the pendingBuild promise was set and then removed after the build is done.