Path Expression parser fails for Expression References with relative paths (unless starting with the parent path element '..')

[danlea]

Constructing an Sdf.PathExpression with the following Expression Reference strings fails with parser errors:
"%child:collection"
"%./child:collection"

Starting the path with the parent path element '..' works around this bug, but requires the base Prim name:
"%../base/child:collection"

The use of "./" in a PathPattern (i.e. outside of an Expression Reference) also causes a parser error:
"./child"

Note that the expected result of Sdf.PathExpression("%child:collection") can be constructed as follows:

expressionRef = Sdf.PathExpression.ExpressionReference()
expressionRef.path = "child"
expressionRef.name = "collection"
pathExpression = Sdf.PathExpression.MakeAtom(expressionRef)
print(pathExpression.GetText())

%child:collection

While these expressions will be made absolute when used in an Path Expression Attribute, path expressions provided as strings (through API or in usda) should accept all of these examples.

Reproduced with OpenUSD 24.05 and 24.08.

[jesschimein]

Filed as internal issue #USD-10614

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