Treat opaque file URIs as errors (#1087)

Opaque file URIs are URIs whose scheme-specific part does not start with `/`.
For example, `file:foo/bar.txt` is an opaque URI.

Currently, this has the unintentional behavior of: look for file `foo/bar.txt` from the process working directory.
These are effectively dynamics imports; from a single import, we can't statically analyze what it resolves as.

According to RFC-8089, File URIs must have paths that start with `/`. So, these are actually _not valid URIs_.
See the grammar defined in https://datatracker.ietf.org/doc/html/rfc8089#section-2

This changes Pkl's behavior so that these URIs are treated as errors.

pkl-core/src/test/files/LanguageSnippetTests/input/errors/invalidFileUri1.pkl

@@ -0,0 +1 @@
+res = read("file:path/to/foo.txt")

pkl-core/src/test/files/LanguageSnippetTests/input/errors/invalidFileUri2.pkl

@@ -0,0 +1 @@
+res = read*("file:path/to/foo.txt")

pkl-core/src/test/files/LanguageSnippetTests/input/errors/invalidFileUri3.pkl

@@ -0,0 +1 @@
+res = import("file:path/to/foo.pkl")

pkl-core/src/test/files/LanguageSnippetTests/input/errors/invalidFileUri4.pkl

@@ -0,0 +1,9 @@
+// Ideally, this should have the same error message as `invalidFileUri2`; the error is that the glob
+// pattern is invalid.
+//
+// But, this is somewhat challenging to fix, because the `URISyntaxException` gets thrown before
+// `ImportGlobNode` is initialized.
+//
+// Regardless, this error is good enough (given this error message, users know what to do), and we
+// can afford to be pragmatic here.
+res = import*("file:path/to/foo.pkl")

pkl-core/src/test/files/LanguageSnippetTests/output/errors/invalidFileUri1.err

@@ -0,0 +1,13 @@
+–– Pkl Error ––
+Resource URI `file:path/to/foo.txt` has invalid syntax.
+
+x | res = read("file:path/to/foo.txt")
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+at invalidFileUri1#res (file:///$snippetsDir/input/errors/invalidFileUri1.pkl)
+
+File URIs must have a path that starts with `/` (e.g. file:/path/to/my_module.pkl).
+To resolve relative paths, remove the scheme prefix (remove "file:").
+
+xxx | text = renderer.renderDocument(value)
+             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+at pkl.base#Module.output.text (pkl:base)

pkl-core/src/test/files/LanguageSnippetTests/output/errors/invalidFileUri2.err

@@ -0,0 +1,12 @@
+–– Pkl Error ––
+Invalid glob pattern `file:path/to/foo.txt`.
+
+x | res = read*("file:path/to/foo.txt")
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+at invalidFileUri2#res (file:///$snippetsDir/input/errors/invalidFileUri2.pkl)
+
+Scheme `file` requires a hierarchical path (there must be a `/` after the first colon).
+
+xxx | text = renderer.renderDocument(value)
+             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+at pkl.base#Module.output.text (pkl:base)

pkl-core/src/test/files/LanguageSnippetTests/output/errors/invalidFileUri3.err

@@ -0,0 +1,13 @@
+–– Pkl Error ––
+Module URI `file:path/to/foo.pkl` has invalid syntax.
+
+x | res = import("file:path/to/foo.pkl")
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+at invalidFileUri3#res (file:///$snippetsDir/input/errors/invalidFileUri3.pkl)
+
+File URIs must have a path that starts with `/` (e.g. file:/path/to/my_module.pkl).
+To resolve relative paths, remove the scheme prefix (remove "file:").
+
+xxx | text = renderer.renderDocument(value)
+             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+at pkl.base#Module.output.text (pkl:base)

pkl-core/src/test/files/LanguageSnippetTests/output/errors/invalidFileUri4.err

@@ -0,0 +1,13 @@
+–– Pkl Error ––
+Module URI `file:path/to/foo.pkl` has invalid syntax.
+
+x | res = import*("file:path/to/foo.pkl")
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+at invalidFileUri4#res (file:///$snippetsDir/input/errors/invalidFileUri4.pkl)
+
+File URIs must have a path that starts with `/` (e.g. file:/path/to/my_module.pkl).
+To resolve relative paths, remove the scheme prefix (remove "file:").
+
+xxx | text = renderer.renderDocument(value)
+             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+at pkl.base#Module.output.text (pkl:base)