Account for ambiguity in matching templates by incorporating template priority order from core

[westonruter]

This fixes a bug identified in https://wordpress.org/support/topic/amp-notice-on-woocommerce-product-search-page/

It turns out that if you have a custom post type registered (say product), this custom post type has an archive, and you do a search among the custom post types with a URL such as:

?s=foo&post_type=product

The resulting WP_Query has both is_search and is_custom_post_type set. WordPress will end up serving the search.php template here because the is_search() condition comes before is_post_type_archive() check in template-loader.php. Nevertheless, we were not taking this logic into account but are rather just issuing a warning when more than one template is matched:

amp-wp/includes/class-amp-theme-support.php

Lines 577 to 593 in 3a2064e

	/*
	* If there are more than one matching templates, then something is probably not right.
	* Template conditions need to be set up properly to prevent this from happening.
	*/
	if ( count( $matching_templates ) > 1 ) {
	_doing_it_wrong(
	__METHOD__,
	esc_html(
	sprintf(
	/* translators: %s: amp_supportable_templates */
	__( 'Did not expect there to be more than one matching template. Did you filter %s to not honor the template hierarchy?', 'amp' ),
	'amp_supportable_templates'
	)
	),
	'1.0'
	);
	}

Now that we see there is a case where template conditionals can be ambiguous, we need to incorporate the priority order of template conditional checks in order to make sure we select the same matching template as WordPress core will do.

The one additional consideration that the AMP plugin has to make here is the case where a site registers a custom template type that is outside of the WordPress template hierarchy entirely. Since such a template type would not be accounted for in the template-loader.php conditional, in this case we just use the custom matching templates by assuming they are more specific than the ones coming from core.

This PR also fixes an issue with the build process where compiled files from other branches (e.g. amp-stories-redux) were not getting cleaned up and were unintentionally being included in the builds. So now those compiled files are all removed before invoking Webpack when doing a build.

Build for testing: amp.zip (v1.1-alpha-20190307T202052Z-bef18cf6)