Account for multiple potential sources in @glint/transform

[dfreeman]

This PR is step 1 toward supporting standalone templates (#22). The primary change here is eliminating implicit assumptions about a 1:1 mapping between inputs and outputs in the transformation process.

This is reflected in a few key places:

• APIs that deal with source-to-source mapping now require specifying which input a source location is tied to for input -> output mapping, and provide that same information for output -> input mapping
• Errors emitted from the transformation process now include a reference to the source where the error originated
• ReplacedSpans are now CorrelatedSpans with source attribution, and cover the entire transformed file, rather than only portions that have been rewritten. This simplifies the process of answering offset mapping queries, and gives an explicit answer to the question "where did this content originate" for any location in the transformed output.

Further updates will be required to:

• actually incorporate the contents of standalone template files into the transformed module
• handle template-only components
• teach @glint/cli and @glint/tsserver-plugin how to discover and observe template files