while I was working on #578 - which is completely based on "Rules for each purl component", i found some cases that are not properly parsable with the current specced "How to parse a purl string in its components"

non-canonical test case, valid according to spec :

• pkg://////foo-type/////bar-namespace////bazz-name/////@version
  • multiple slashes after the ":" -

    purl-spec/PURL-SPECIFICATION.rst

    Lines 131 to 132 in c53ba0e

    	followed by one or more slash '/' characters, such as 'pkg://', and MUST
    	ignore and remove all such '/' characters.

  • multiple slashes leading and trailing the namespace -

    purl-spec/PURL-SPECIFICATION.rst

    Lines 149 to 150 in c53ba0e

    	- All leading and trailing slashes '/' are not significant and SHOULD be
    	stripped in the canonical form. They are not part of the ``namespace``.

  • multiple slashes leading and trailing the name -

    purl-spec/PURL-SPECIFICATION.rst

    Lines 168 to 169 in c53ba0e

    	- All leading and trailing slashes '/' are not significant and SHOULD be
    	stripped in the canonical form. They are not part of the ``name``.

• ... add other cases here ...

parsing pkg://////foo-type/////bar-namespace////bazz-name/////@version

1. split once from right #
   • no subpath
   • remainder: pkg://////foo-type/////bar-namespace////bazz-name/////@version
2. split once from right ?
   • no qualifiers
   • remainder: pkg://////foo-type/////bar-namespace////bazz-name/////@version
3. split once left on :
   • schema: pkg
   • remainder //////foo-type/////bar-namespace////bazz-name/////@Version
4. strip leading/trailing slashes
   • result: foo-type/////bar-namespace////bazz-name/////@version
5. split once left of /
   • type: foo-type
   • remainder: ////bar-namespace////bazz-name/////@version
6. split once from right on @
   • version: version
   • remainder: ////bar-namespace////bazz-name/////
7. split remainder once from right on /
   • remainder: ////bar-namespace////bazz-name////
   • name: `` (empty string)

looks like the following instructions is placed at the wrong position:

• Strip all leading and trailing '/' characters (e.g., '/', '//', '///' and
  so on) from the remainder

instead, at this position, it would be enough to strip leading slashes,
and in addition, the strip of leading and trailing slashes is needed for name/namespace