Support dynamic enable/disable of classpath entries

[laeubi]

Currently it is not possible to have JDT dynamically enable a classpath entry based on some system state, this might include:

• The os / ws / arch of the running system (e.g because some code is targeted for a special system, see Support compile code for each environment separately eclipse-tycho/tycho#632)
• The project JVM (e.g. because some code is targeted only for a certain java version see [JEP 238] Multirelease support #265)

My proposal therefore would be to have a way marking a classpath entry as disabled, and then related code can handle it as if it is not defined at all (e.g. ignore it during compilation / indexing / ...).

This concept also is generic enough to apply it to any classpath entry type, even though most likely only a few would make use of it (as a first start, only CPE_SOURCE)

[laeubi]

After thinking more about it I'm probably on the wrong track here regarding my initial idea of enable/disable to support:

• Support compile code for each environment separately eclipse-tycho/tycho#632

and the parts for

• [JEP 238] Multirelease support #265

are better solved as you described here:

• Support dynamic enable/disable of classpath entries #614 (comment)
• [JEP 238] Multirelease support #265 (comment)

so I'll close this for now and open another proposal in favor of this for the multi-platform compile.

Originally posted by @laeubi in #614 (comment)