RFR: JDK-8200337: Generalize see and link tags for user-defined anchors

[Hannes Wallnöfer]

Please review a a new feature to allow `@link`, `@linkplain` and `@see` tags to link to user-defined URL fragments in the generated documentation (including in auxiliary `doc-files` documentation).

The changes in module `jdk.compiler` are mostly cleanup changes retained from earlier versions of the patch. The current proposed version uses a very simple change in `ReferenceParser` to avoid parsing the member name section of the reference when a non-member fragment is encountered.

The implementation almost exactly follows the plan described in the JBS issue, except that it supports both ways to mark a user defined fragment:

 - a fragment name containing a `-` character
 -  a fragment starting with double `##`
 
The reason for supporting both ways is that the first option practically imposes itself ('-' not being a valid character for member names and auto-generated header ids making use of this), while the second is necessary to support the existing `id` attributes in JDK and other sources which should be usable with this feature without needing to be changed.

Links to auxiliary documentation files follow the same rules. They are recognized by looking for `/` characters in the fragment name. This means that ordinary `id` attribute values must not contain `/`, while auxiliary file paths must contain a `/` character. Both restrictions should be easy to sustain.

One thing that is difficult for this feature is to provide a good link label if no label is supplied in the tag. In contrast to program element names a fragment name does usually not make a good human readable name. The solution is to use the fragment name as default label text. I expect that the feature will usually be used with a user provided label.

The CSR for this feature is still unfinished, I hope to get it ready for review within the next few days.

-------------

Commit messages:
 - Remove trailing whitespace
 - Only support double hash as fragment/path indicator
 - JDK-8200337: Generalize see and link tags for user-defined anchors

Changes: https://git.openjdk.org/jdk/pull/10395/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10395&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8200337
  Stats: 388 lines in 10 files changed: 333 ins; 16 del; 39 mod
  Patch: https://git.openjdk.org/jdk/pull/10395.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10395/head:pull/10395

PR: https://git.openjdk.org/jdk/pull/10395