The upstream interpretation of `$civicrm_paths['civicrm.packages']['url']`
currently requires a trailing slash...  but `autoload_civicrm_asset.php` may
not provide trailing slashes depending on the happenstance of
defaults+customizations.

This patch updates the generated values for `$civicrm_paths` and
`$civicrm_asset_map` so that they consistently use (or do not use) trailing
slashes.

(drupal8-clean) The JS/CSS assets are sync'd to web dir, but the $civicrm_paths should still autoload PHP from the src folder.

This improves support for drupal8-clean:

1. The content of `$civicrm_paths['civicrm.packages']['path']` should be the
   *source*, because that has all files (including PHP and JS/CSS)
2. Fix generation of quasi-relative paths

There is no perfect file-mode:

* `copy` is very compatible, but it's annoying for development. (You have to republish
  after every change.)
* `symdir` is very convenient for developyment, but it's not good for production.
  (You rarely have to manually publish -- but it exposes extra files to the web.)
* `symlink` is middle-ground. (It doesn't expose extra files, and you may have to
   republish more times, but not as much.)
* Both `symlink` and `symdir` depend on the platform, filesystem, and HTTPD options.

This patch allows the following recommendation:

* Set the default to `copy`, and use that default for production/staging/CI.
* On local dev machines, set CIVICRM_COMPOSER_ASSET=symdir so that you never
  have to tweak the configuraiton for a particular build.

Ex: `composer-extra-files` downloads CSS assets in
`vendor/civicrm/civicrm-core`; then `civicrm-asset-plugin` copies them to
`web/libraries/civicrm/core`.

USE_TEST_PROJECT - Document it. Remove KEEP_TEST_PROJECT, since the use-case is handled equally well by USE_TEST_PROJECT.

This is a subtle change with two main effects:

1. Extension folders are named by the *extension-key* rather than the *composer-package*.
   This should make it easier to setup runtime rules for properly constructing links to
   the extensions' assets.

2. All Civi-related assets go to same folder, regardless of `<vendor>`. This makes it
   easier to cleanup orphaned assets (e.g. `rm -rf web/libraries/civicrm && rebuild`).