fix: use parent xml to parse child

[shetzel]

What does this PR do?

Reuses the parsed custom labels file when parsing the children to improve conversion performance.

What issues does this PR fix or reference?

forcedotcom/cli#1194
@W-9990964@

Functionality Before

converting CustomLabels would take a long time since it would read and parse the CustomLabels.labels file for each label (child) within the file.

Functionality After

The CustomLabels.labels file is read and parsed once, then reused to parse the children.

Testing Notes:

• Run the source plugin label NUTs with this branch linked and ensure everything passes.
• Manually test conversion (force:source:convert) of all custom labels using sourcepath, metadata, and manifest flags. Use a project with lots of labels and multiple package directories. Ensure the converted file contents and package.xml looks correct.
• Test converting individual labels, both from within the same package directory and within multiple package dirs. Ensure the converted file contents and package.xml looks correct.
• Test converting CustomObjects and ensure nothing has regressed
• Test converting other non-decomposed types with children such as Workflows or AssignmentRules

[WillieRuemmele]

QA notes:

• converts CustomLabels across MPDs, single packages
• plugin-source NUTs pass
• 8k convert 10mins -> 13s
• CustomObjects work
• other nonDecomposed types (AssignmentRule) work

[mshanemc]

why keep this around at the class level if it's only used inside one recompose method? It could be kinda big for those large-number-of-labels scenarios.

[shetzel]

It should be really small actually. In the case of custom labels there would only be 1 entry per CustomLabels.labels-meta.xml file, which is only 1 per package directory. This could possibly be moved to within the recompose function but I wasn't sure if there were situations where multiple parent CustomLabels would exist in a ComponentSet so to be safe I added this as an instance var.