Revisit directory structure of components project

What does this MR do and why?

Related to #377327 (closed)

In Define CI components syntax (#377327 - closed) we discussed a lot about alternatives to the directory structure. Some of the issues highlighted were:

• more explicit file names
• component filename to affect the component path

Given a components project gitlab-org/dast, in this MR I'm proposing the following changes:

• the default component component: gitlab-org/dast@1.0 is identified by the file default.template.yml (or default.step.yml depending on the context/type).
• the explicit component component: gitlab-org/dast/api-scan@1.0 is identified by the file api-scan.template.yml (or api-scan.step.yml depending on the context/type).
• the components project contains only 1 README.md file at the root directory. This means that as we version and publish the component as a whole we can also document it as a whole. Like CircleCI Orbs. No need for granular README.md files per component.
• Adjacent files can still be supported (e.g. for steps components) inside a <component-name>.<type> directory.

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